Took the car to a decently respected shop (Ecotech) in STL for a new clutch and exhaust work. Went there today and asked to hear the new exhaust. The exhaust sounds good when revving up but once the gas Pedal is let out it sounds like the exact same rattle that comes off the BMW N54 turbos when it’s wastegates starts to fail. While there the shop also took the plenum off to do a valve cover gasket job and EGR delete. Is it possible they didn’t hook up a vacuum hose correctly or missed one all together? It is important to note this car was in very good running shape when it went in besides having a slipping clutch and a slight oil leak. 1991 TT, Manual with 93,000 miles with stock turbos. I had them put on Z1 downpipes and Z1 test pipes along with the cat back I already had. Any help would be very appreciated because I can’t find anything else on the internet that is similar to what I’m experiencing with this 300zx.

I know exactly the sound you are referring to (have an N54 E90), I recently fixed it by tightening the actuator arm and hoping it'll stay fixed. There are so many parts that could rattle how you describe, it could be anything from the actual wastegate to the new downpipes rattling against something, or a heat shield somewhere that is not secured properly. It's too hard to tell.

Hey thank you very much for the reply. So if the vacuum hose came off of the waste gate that would just keep the wastegate from releasing boost correct? That would not cause the waste gate to rattle like an N54. And yes I’m sorry you have had that issue. My mother had one and unfortunately we had it occur on her car. And this is oddly acting the exact same in every way. So if you had to guess since the wastegates were silent before the work the culprit is probably something rattling on the downpipe or a heatshield?

The wastegates on the N54 are modulated 100% of the time via an ECU controlled vacuum solenoid, they don't act like oldschool wastegate actuators which rely on a spring to keep them closed and only open when boost is applied. Basically the N54 wastegate actuators work opposite of most standard actuators thus if the vacuum line to the actuator on the VG fails the wastegate stays closed unless the exhaust flow from the cylinder forces it open, which shouldn't happen unless you have a very worn spring. What this does is makes the engine susceptible to overboost if the vac line comes off. On the N54 the spring actually works to keep the wastegate open so if there is ever an electrical or mechanical failure in the control system there is no chance of overboost (unless the wastegate gets stuck closed of course). Wastegate rattle is a pretty easy fix on the N54, it just takes tightening the wastegate arm a couple turns though I don't know how long that fix lasts (the dealer will try to sell you an entire turbo). I guess if the spring in the actuator collapsed or broke then it could act the same on the VG, easy way to tell is to put your hand on the actuator arm and wiggle it, if there is no wiggle then its not the actuator. HARD PART is getting to the arms, you may be able to reach the driver's side from under the car since it's outboard of the turbo but on the passenger side the actuator lives between the turbo and the block so you need to have some very bendy little hands to get in there.

So it turns out what I was hearing was the rasp from the downpipes and the test pipes. Because I was in the car it sounded like something mechanical rather than exhaust related. As soon as I got out of the car and had someone else rev it I could hear everything was relatively normal. Sorry for wasting your guys time on what turned out to be a non-issue. But also for everyone going from stock precats and cats to aftermarket downpipes and test pipes. Realize your car may sound like this from inside the cabin.

Thanks again!

Glad you got it figured out, and thank you for coming back and actually closing the loop by posting a definitive resolution.