KHedges

Something under the rear passenger seat is thumping against the underbody when I go over bumps or make left turns. Originally I thought it was the loose end of the sway bar disconnected by Previous Owner when he lowered it (badly). It was also not the ABS sensor cable bracket rattling against the lower suspension arm, as zip-tying it in place didn't change the noise. (It sounds like something with more mass than a cable, too.)

I have a known issue with dead rubber in the driver's side engine mount that causes vibration when idling.

I didn't see anything loose under the car, nothing moved when I pushed on it, and I secured everything in the spare tire well when replacing my dead donut spare with an almost-dead old tire/wheel.

However, we were a little creative when replacing the catalytic converters. We used one from my 2.8L B5 Passat wagon, but the other one from the Passat was also dead, so we found an affordable CARB certified aftermarket cat. There's a section where the pipes from the catalytic converters go back to a box thing before separating to the twin mufflers and exhausts... and on one side, there's about 1" clearance between the side of the indention in the floor pan but on the other side it's right up against the waffle heat shield. I have been saying for all of 2018 I think it's something to do with our cat installation, my friend's been insisting it's the sway bar, and I think this might be the second time since June 2016 when I bought the Passat I will be right about something. (I'm really inexperienced and he's been working on his fiancee's 2.8L B5 Passat, other VW cars, and racing since he was 16 which is at least half his life.) The other time I was right about something was when I had more time to sift through forums when we were installing the timing belt and the cam chains didn't seem quite right--because of the typo in Bentley. Thank goodness for enthusiast forums! (I should also check our A4 B5 and the PassatWorld B5 forums for this issue. This is the kind of thing all the models using the 2.8L V6 could have in common.)

Has anyone else had this noise issue and solved it? Friend says that "if it's periodic, it's likely to be the drive train; if random, something else. Good luck looking for it!"

I'm hoping it's an exhaust pipe issue that can basically be handled like plumbing, instead of some death rattle in the rear differential. (This is my first AWD car so I am not at all accustomed to the complication of having a second set of drive wheels; it's been FWD for me from 1991 to 2018.)

If anyone also has a B5 Passat, we parted it out...

clancy

KHedges

Thanks! I may not have been clear that we replaced the rear shocks and coils, but I can see that if the rest of the system was that far out of spec, it could be permanently damaged. I don't know how long Previous Owner had been trying to operate it that low.

I think the exhaust is the most likely culprit. Where could I get good pictures of what it should look like under the car? Or should I try to find one in good shape at a junkyard to compare? I have little experience and don't always know what I am looking at. (I didn't see everything from all angles when I was helping part out my old Passat. And the further you get from the engine, the more different it is from an A6.)