Strange noise, only under load...

[Jonathan]

Ideas welcome...

Within the last couple of weeks I have heard a strange noise coming from over my shoulder when driving the car. It seems like it is coming from somewhere in and around the engine bay.

It has gotten more pronounced in the last few drives and has me wondering/worried more about what is going on.

Here's what I can tell you:

The noise is somewhat of a vibration or rattle. A flutter perhaps. You ever hum or buzz with your lips and if you do it just so, you get a tickle? That's what sense I get with this noise. Maybe metallic, maybe air.
It comes and goes only while pressing the gas pedal. And repeatable too. It won't make the noise when idling, only when pressing the gas. More so when pressing initially on the gas, or letting it off, but not so much as you settle in on a constant gas pedal pressure.
I cannot get it to repeat when parked in the garage, only when the car is in motion.

Makes me think it is RPM/load based and perhaps also related to the wheels in motion.

I thought perhaps it was a piece of my muffler heatshield rubbing against the muffler peg as you put the engine under more load. This is kind of what the sound made me think of first, but I looked over the heatshield and I'm not sure this is it. The end I thought could be rubbing, I put a small piece of foam tape just to test and the noise continued.

Next thought was a fuel injector seal leaking as I thought perhaps the noise was air rushing by in some manner. A fellow owner mentioned that since it happens only under load and not at idle, that's the opposite time to when you might hear an injector seal leaking, i.e. more likely at idle and not under load when the vacuum is stronger. I don't smell much fuel in the engine bay either. Not strong anyway, faint every so often perhaps but not super noticeable.

I stopped by a shop to talk to the guy about some work on my Pontiac and asked him his thoughts on the noise. He asked me whether it does the noise when stationary versus moving as I hadn't tried that yet. I did the test and I could not get the noise to repeat when sitting in one spot, only while moving. He mentioned how if the rear differential was quite low on oil, it might be subjecting the "roller bearings" to wear and that noise could be them wearing out. I'm not familiar with the internals of the rear differential, but since the noise only happens when moving, I agree it's likely related to the wheels in motion and not an idle engine. That might rule out the A/C belt or idler pulleys going bad. It might also be the alternator giving up if the bearings in it are going bad, but again, since it will not do the noise parked, I am thinking the rear diff idea might be the source.

Does any of that sound familiar to any of you? Any other ideas of what this could be or where it might be coming from? Any ideas for tests to do?

I can pull the A/C belt off pretty easily to rule that out if needed. I could even remove the muffler heatshield and all the brackets temporarily to see if it matters.

I can also see about opening up the fill port on the rear diff and checking the level. I have not checked this recently, not since refilling perhaps 3 seasons ago. Likely from the poor access to the fill port. Poor excuse, yes, but that's the truth. My car has been dripping some oil for a while, but have not been able to determine exactly where it is coming from. It is reddish but not pink like ATF. My ATF cooler hoses don't appear to leak and neither does my auto trans pan or dipstick. The drips that come appear right under the frame cross member which others have labelled as a main seal leak on the engine. Engine oil level has been fine each time I check it via the dipstick. Same for the ATF level via dipstick. Car is shifting fine and the performance has been good all season. If this noise wasn't there, you wouldn't suspect a single thing is wrong with the car at all.

Thanks in advance for any insight.

[opethmike]

Is it loud enough that you could possibly record in the car while driving?

[Jonathan]

Is it loud enough that you could possibly record in the car while driving?

I can try doing that. I have a couple other places to check on spots things might be rubbing against each other as well. It's almost like when you stand on the gas a little and the engine "flexes" under that big PRV power, something just comes into contact with another nearby surface and rubs/rattles and then stops once you ease off the gas. Not sure that's what is going on, but it's along those lines.

[Bitsyncmaster]

I had a buzzing sound come and go a few years ago and it sounded like it was coming from the drivers side door (left shoulder). Never could pin point it with any conditions. I've not heard it since. I actually forgot about it so not sure if anything I've fixed took care of it. I really think it was some wind noise in the top of the door.

Come to think of it, I did remove my top door seal because it would always fold over and push the T panel up when the door was closed.

[Jonathan]

I have found one fastener not in place on my muffler heatshield.

If you look at the right hand side lower bracket... http://store.delorean.com/p-6469-bra...rh-recond.aspx

See the rectangular extension sticking out sort of on its own? It corresponds to a hole in the heatshield that's sort of in the middle... and not exactly accessible to get a bolt through to thread into the nut that's welded onto the bracket... because there's a muffler in the way!! Who thinks this stuff up??

Anyway, that slim rectangular section of bracket is all that is keeping the right side of the heatshield secured to the bracket, otherwise this "flap" could be the culprit for my vibrating noise. Maybe. Who knows. I put some double sided 3M tape on it to go for a test drive to see if that changes anything. Although getting a bolt back into place on the heatshield wouldn't be a bad idea either.

[Jonathan]

Ok, so it isn't that extension of heatshield not well secured to the bracket. At least the attempt to stick the two together and stop them from vibrating didn't change the noise.

I did not yet get a chance to record it, but it's still there. Since it is so correlated to the accelerator being pressed, I am thinking it's possible it's related to one of the moving pieces in the fuel system. Like the idle speed motor rotating or the fuel distributor stem going up and down. Maybe one of those is sticking or whining or something and making this whistle noise business. I kept it in mostly the neighborhood on this last test drive and the noise is more pronounced when you are easing/coasting towards a stop sign and then give it some gas just to get you up to the line. Like if it is idling and then you give it a bit more gas at that low speed, the noise is louder, like this is a spot where something is working harder to do whatever it is doing.

Man these cars are silly. Fubar.

[David T]

Tracking down noises can be extremely difficult. They may sound like they are coming from one place yet are actually coming from another. Check the upper right muffler bracket and go over the whole exhaust system. Check for exhaust manifold gasket leaks. Look at anything thin and metal like covers and such.

[DMC-81]

Hi Jonathan,

I would recommend verifying the level of the differential oil before driving it too much further, just in case it is the carrier bearings.

[Jonathan]

Tracking down noises can be extremely difficult. They may sound like they are coming from one place yet are actually coming from another. Check the upper right muffler bracket and go over the whole exhaust system. Check for exhaust manifold gasket leaks. Look at anything thin and metal like covers and such.

Thanks David. The cross over pipe cover that sits above it is one I'm going to get a better look at to see if something is rubbing.

Hi Jonathan,

I would recommend verifying the level of the differential oil before driving it too much further, just in case it is the carrier bearings.

I agree with you Dana. If it turns out to just be something annoying, but not damaging, so be it. But in the meantime, I like the reassurance of knowing where that oil level is. I may check the level, drain it and refill it anyway while I'm under there.

This is a photo of the drip tray I put under the car each day. I tend to clean it off before putting it back under after a drive.

IMG_5078.jpg

You can see on the left where I dripped brand new ATF as it is bright pink.

The clearish, yellow drip on the lower edge is brand new gear oil AKA rear differential fluid.

The top clearish, yellow drip is brand new engine oil, Castrol 20W50.

The long darker smear on the right side is what accumulated under the car while it dripped from about the rear cross member of the frame (slightly forward of that).

I did this little colour test because the drip oil doesn't look anything like the others. Granted the oil gets dark and gross while in the engine over time, but it still didn't really help me determine exactly what was dripping.

[DMCMW Dave]

Yeah - these can be hard to find. Almost easier to eliminate things:

- Wheel bearings generally cause a growl or vibration that can be made stronger or weaker by making a right or left turn. Not acceleration.
- Low differential fluid will cause gear whine way before the bearings get noisy. The whine will generally be only on acceleration or deceleration. Once you have it you're stuck with it, putting the oil back in won't fix it.
- I've seen vibration under acceleration caused by bad motor/trans mounts, i.e. on acceleration the engine actually rotates slightly opposite the wheel rotation direction. This kicks the transmission up against the frame, and you get the feel of the engine through the frame into the seat of your pants. You may have noticed that on EVERY car there is an area of the frame just forward of the transmission that looks like someone beat it with a hammer. I believe this is an assembly line "hack" to fix this problem. Never seen it documented but you see it on every car. This motion also kicks the other end of the engine down, which can make the exhaust hit things that it doesn't normally hit. Look for telltale marks on brackets.
- If you had described it as more of a miss, I'd suspect plugs/wires/cap/rotor. But that's not usually as subtle as you describe. It's not likely the fuel system since the fuel system isn't "timed". It could be a leaky injector seal, but that's pretty unusual to be had enough to feel it like that.

Question - When you say "on the gas" do you mean when acceleration only or do you feel it at a steady cruising speed?

Does it feel like a ignition miss (i.e. power cut out at engine RPM)? OR faster than that? To simulate an engine miss, take off a plug wire. Don't leave it like that for long, hard on the converter, but a few trips around the block won't hurt anything.