Thump sound coming from exhaust in certain conditions

[laxshman]

Hey all, I've decided to make a thread because I can't find anything similar around nor any mention in the FAQ.

If I drive my E36 around town for short bursts it's fine, but if I hit highway speeds for 20 minutes or so the car will begin to thump and vibrate from the exhaust.
This continues until I come to a stop and idle for 5 minutes or so, after which it returns to normal.
The car is happy to rev without vibration when idle or in neutral, Giubo is brand new and the tail shaft was just replaced with a used one.
It's not the heat shield banging around because the sound is present with and without the heat shield installed.
I'm not sure what to make of it as it is clearly the exhaust thumping, but I am not sure why or where it is coming from.
I've just replaced two manifold gaskets but to no avail.

I've uploaded two short videos to demonstrate the sound

https://youtu.be/w9YDS79LjZo - no thumping sound, everything nominal

https://www.youtube.com/watch?v=LyNfLcvbJnc - thumping sound present, unsure where it could be coming from.


Cheers for reading

[LockDots]

I don't hear much of a difference honestly. The second video sounds like the exhaust is a little more prominent, but not something I'd worry about. Sounds like the engine is just running a little differently once it's been running a while.

How many miles on the car and have you changed the front O2 sensors?

[laxshman]

I've done about 7,000km in the car, the sound isn't the trouble at the moment, rather the fact that it rumbles/reverberates off its tits once you hit get up to speed. Exhaust thumping is the only symptom that sticks around after you slow down.
I'll look into O2 sensors

[LockDots]

I've done about 7,000km in the car, the sound isn't the trouble at the moment, rather the fact that it rumbles/reverberates off its tits once you hit get up to speed. Exhaust thumping is the only symptom that sticks around after you slow down.
I'll look into O2 sensors

Well, it's hard for us to sense the thumping via audio lol that's why I mentioned it. I mentioned the O2 sensors because my car had a somewhat rough idle with my old ones (about 109k mi on them) and it smoothed out drastically with new ones. I got NTK sensors btw.

[flyfishvt]

Shot in the dark here but I had a broken stud on one of the muffler hangers and it caused a wicked thud. Wait for the exhaust to cool off and grab the tail pipe. Yank it up and down and side to side. Don't be gentle. If you hear any noise then jack up the test and inspect the hangers.

There is also an exhaust hanger that bolts to the transmission. It's rare to find one that isn't rotted out

[laxshman]

That could be it. I've just had a look and both of the rear muffler hangers are half torn, while one of the centre hangers is missing entirely, in its place is a zip tie. Classic.

[laxshman]

No luck with anything yet. I'm thinking maybe the vanos is cranky or the spark is dodgy.
Here's a better video of the issue: engine or exhaust sounds really bubbly. Like it's gargling mouthwash or something.

https://youtu.be/VJdMaTyVWEo