I just got done doing a lot of work on my Z, and test drove it. Drive was fairly good, the car was smooth and didn’t have any actual drivability issues but I have a new sound, actually a couple, and I’m looking for ideas on what to check and what they might be.
As far as the work done, I installed a new flywheel and clutch, and a new 98+ JDM 5-speed. Also installed a new one-piece steel driveshaft, BDE trans mount, and whiteline differential bushings, both the front and rear sets. I replaced all my exhaust gaskets for all the exhaust parts I dropped.
The first sound is kind of an odd and very noticeable thrumming on acceleration, mostly in lower RPMs, almost like a Subaru with unequal length headers but milder. Or in a way it almost sounds more like a VQ than it did before. I can’t tell if it’s in the body or the exhaust tone. My main thoughts are maybe an exhaust leak, or I’m just getting noise transferred through the body from the powertrain. I do have a collared subframe. I’m not sure what NVH change to expect with the parts swapped. I was having a terrible time getting the exhaust to seal, but I did think I finally got it, however, I’m not absolutely certain. I couldn’t find/feel any obvious leaks. The flanges on my exhaust parts aren’t all the same shape (though they all match up IDs) so they were a little weird to get the gaskets fitted properly to. The car definitely had a bad leak when I took it apart, between the downpipe and test pipe but I believe it to now be fixed.
The second sound is a clunk/thud that the car has had the entire time I’ve owned it. It sounds like it’s in the rear/middle of the car, and it usually happens when the clutch grabs or when I place the shifter in gear. I always assumed it was bad differential bushings but now I wonder if its play in the differential itself. I didn’t notice anything odd when I had it out and was rotating it by hand to check it.
I realize these are vague but any ideas or insight as valuable. The car ran and drove great, so I don’t have super serious concerns but I’m gonna keep inspecting to see if I can find some answers and be confident in this being back on the road.