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.