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Thump sound under car.

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Hello everyone I have a 94 325is e36, I have a weird thump sound that happens when I'm at a stop and engage to first gear. Not only that but also let's say I'm in any gear and driving normally at any mph and let go of the gas pedal for a couple of seconds then step on it again to remain speed it also does the thump sound every single time I disengage and engage the gas pedal. It bearly started to do this like about a week ago.

I changed out the entire driveshaft with new csb about 5 months ago. I was thinking it was possibly the guibo (flex disk) since it was still in ok shape when I did the driveshaft so I had left it alone.

Today I jacked the car up and saw that the guibo was cracking so I went and purchased a new one took everything apart under the car put everything back together and the thump sound is still there. It didn't minimize it at all or make it louder, it sounds the exact same as it was prior to me doing the work. Maybe my transmission is going already??

Any thoughts would be greatly appreciated. Thanks.
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Anyone??
I would think u joints on the drive shaft?

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Check your differential, bushings are either shot, or your bolts are loose.

Jack the car up and "drive" it. Have someone look if your diff moves.

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Ive got the same thing. I knew the previous owner of my 325 and remember it doing that for the last few years. Only on first gear though during the shift into it. Not on the accelarator. That sounds like the three aformentioned bolts that hold the diff. My e30 broke one of those after the middle one went loose. It was quite a mess.
I don't think joints are done because the driveshaft was new when put on...another thing I didn't check was my motor mounts. I will do that tomorrow along with my diff bolts. I will post as soon as I get to the root of the problem thx to all.
Okay so I found my problem, first off my left motor mount was broken, I replaced both left and right motor mounts today. Second, one of the differential bolts that is like right underneath the driveshaft (holds from diff to subframe) pointing towards the rear of the car was snapped in half :(.

I put in another bolt with 2 washers just for now but it still thumps underneath since it dosent tighten up.

So is it possible to drill out the piece of the bolt that is in the differential and make some new threads for a bigger bolt?

Here are some pictures of the broken bolt, broken motor mount and the flex disk I changed as well that was starting to crack.


Thanks everyone!

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those are a high grade bolt. iirc like 4 $ each. buy one.
I will need to drill out the other piece and get a bigger bolt right?
I knew it! My car did the same thing.


You're going to have to replace it with the stock bolt, as stated.
Did you have the other piece stuck in the diff like mine? By drilling out the stuck piece won't it mess up the threads? Then making the same bolt not tighten up?

Thank you for your help.
I knew it! My car did the same thing.

You're going to have to replace it with the stock bolt, as stated.
No, mine did not break off. Is there any part of the bolt you could get a hold of? A great way is to weld a nut on the end of it. The high heat helps to break it loose, then you loosen the nut and BAM, all fixed.
Here is a pic of the broken bolt. There is a piece of the bolt in the diff still which is the end of the bolt. I wonder how much a shop would charge to do this? Can this be done without removing the diff at all? Or will I have to remove it?
No, mine did not break off. Is there any part of the bolt you could get a hold of? A great way is to weld a nut on the end of it. The high heat helps to break it loose, then you loosen the nut and BAM, all fixed.

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on the 3 cars i have done the driveshaft work on none of the bolts looked like that iirc
So tomorrow I will be dropping the differential to drill out the piece that is broken since I had been busy all week...wish me luck!
Check your subframe mounts. Bad mounts/torn from the body will mimic exactly what you described, sounds like you found your problem, but check them out while your at it, I highly recommend welding in reinforcement plates to all e36 non m chassis.
Problem solved, I drilled the bolt then used an ez out to take out the rest broken front bolt from the diff and put in a new one. Checked out the other mounts and they were all good and not torn or broken. It was a pain! Two of the axle bolts from the left side were stripped from the previous owner. I got a kit to supposedly remove stripped bolts and they didn't work. So I thought outside the box, removed the left axle nut from the wheel and took out the diff with the left side axle still bolted on :). Then put everything back together. No more strange sounds under the car.
glad to hear, but i would have imagined the stripped out bolts would have been far easier to remove with the diff still out of the chassis...:dunno:


regardless, glad to hear the broken bolt came out fairly easily. they don't always...:(




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