Strange thud noise

[DMCNY]

Wondering if anyone has ran across this and might know what it is.

Whenever I first start out with the car and travel for appox. 1/2 mi. I'll get this quick thud noise from what sounds like the rear-center engine area?

It will only happen once and I cannot get it to duplicate the sound ever again no matter what I do until I park the car for a good clip and start out with it again on another day.

I notice it seems to only happen when I shift from 2nd to 3rd and it then takes a second or two after being in 3rd gear before it happens. It's just a quick thud. (sounds like someone took a hammer and wrapped on the fiberglass body quick?)

I've looked everything over pretty well I think. I jacked it up and checked the TAB's, brakes, engine mounts, ect. but cannot find anything wrong or loose.

Any thoughts as to what this may be?

Thanks!

[DCUK Martin]

Does it do it again after you've been in reverse? Possibly loose driveshaft bolts?

I once had a mystery thump and it turned out the battery was slightly smaller than the tray and was sliding about by about 1/4"

[DMCNY]

Does it do it again after you've been in reverse? Possibly loose driveshaft bolts?

I once had a mystery thump and it turned out the battery was slightly smaller than the tray and was sliding about by about 1/4"

Now that you mention that Martin, that makes sence. I back the car into the garage everytime I'm done driving it and never really back up when I have the car out. It only seems to do it after I take it back out of the garage...and I don't have my battery strap on either because I couldn't figure out the latch.

I would have never thought to look there. Thanks so much Martin for the quick responce and I will defintely be checking into that tommorrow when the rain stops.

:cool1:

[David T]

Check the front motor mounts. I found a front, left one broken and it caused a very hard noise to track down. Thuds coming from the front are usually caused by stuff sliding around in the trunk. Also the jack storage area. Sometimes it is hard to figure out where the noise comes from. A noise that sounds like it is coming from the rear is really from the front. With noise complaints you have to start your search all over the car. Check your lug nuts and shocks too.
David Teitelbaum

[DMCNY]

I know this is an old thread, but I figured I'd update it anyway as I have finally found my mystery thud noise. With 100% certainty, it's coming from the heater box. I assume it's a blend door closing. Not really sure if it's supposed to sound the way it does with such a thud noise, but I'm happy I at least tracked it down and nothing is going to fall off the car.

[Bitsyncmaster]

I know this is an old thread, but I figured I'd update it anyway as I have finally found my mystery thud noise. With 100% certainty, it's coming from the heater box. I assume it's a blend door closing. Not really sure if it's supposed to sound the way it does with such a thud noise, but I'm happy I at least tracked it down and nothing is going to fall off the car.

One thing I found. If your trunk adjustment stop stud is screwed in to deep (passenger side), it will hit one of the doors in the heater box. I had to shorten the stud on mine.

You can just remove yours and see it that stops the noise.

[DMCNY]

Thanks Dave, I will check that out.

[Ryan King]

Check the front motor mounts. I found a front, left one broken and it caused a very hard noise to track down. Thuds coming from the front are usually caused by stuff sliding around in the trunk. Also the jack storage area. Sometimes it is hard to figure out where the noise comes from. A noise that sounds like it is coming from the rear is really from the front. With noise complaints you have to start your search all over the car. Check your lug nuts and shocks too.
David Teitelbaum

My Automatic Transmission did the same thing. Check the transmission mounts to make sure they are not cracked in two, and check the rubber mount pieces. My transmission mounts were completely cracked in two. See my photo below... The transmission would kick up when shifting and hit the fiberglass under-body making a loud WHAM noise. Just my experience.

4582

[DMC5180]

He said, it would only happen once per drive not every time the the trans shifted. It's always a good Idea to check the tranny mount brackets regardless.

I would have thought it was the classic rear bearing shifting in the Hub carrier. Mine does this once every time I back out of my driveway while turning the steering wheel to the right. Mine only occurs once per drive too. I only here the the clunk when backing up. Don't really notice anything when I start rolling forward again.

[David T]

Another thing to check when you are under the car is to make sure the half-shafts float. They should be able to slide a little side-to-side. Just grab the half-shaft in the middle and try to move it over. They have to move a little to compensate for the up and down movement of the suspension.
Noises are difficult to fix. It can sound like one thing or coming from one spot and it is really something else you never even think of. Check all of the shock mounts too. Motor/transmission mounts do break.
David Teitelbaum