I'm usually pretty good at diagnosing things, but I'm currently out of ideas and figured I'd reach out to the community.

Symptoms: passenger rear of car makes a loud groaning noise when slowly taking off from a stoplight.

Notes:

-NA FC3S. S4 with clutch type lsd.

-I can't replicate this sound by hand with the car on stands.

-I recently tapped a curb and bent my toe rod. As a result, I replaced every single bushing in the rear of the car with Mazda Comp bushings(dropped the subframe and all), replaced the toe rods, as well as the wheel bearings since I was in there.

-I've replaced rear rotors, pads, and greased the caliper slide pins. It doesn't seem to be a brake dragging issue.

- I did not replace the bottom spherical bushing in the rear trailing arms or the stock vertical dog bone camber links.

-I've disconnected the e-brake cable to test if that was sticking, and the sound still occurs.

-The sound seems to go away as the car warms up or has been driven further.

-Diff fluid was replaced with Redline lightweight shockproof ~10k miles ago.Sound only started occurring recently.

Could the sound be from the diff or from blown stock vertical camber links? Any insight appreciated!

 

You wrote: "I did not replace the bottom spherical bushing in the rear trailing arms or the stock vertical dog bone camber links."

These trailing arm control links, i.e.: "dog bone links", might be your problem. They typically, for some unknown reason, don't have a long lifespan on 2nd gen RX-7's.

 

I noticed when I took mine out that despite the rubber seals they managed to seize up. Not due to rust (though the bolt rusted into the sleeve really well, meaning I had to replace my links), but the spherical bearing in the link just seemed to stick and move jerkily even though it was clean. I'm not sure if yours have stuck, or if there's play in the bearing itself. Mine had no play.

I found on one side that I could pry out the seal, loosen it with PB Blaster, then re-grease with wheel bearing grease and it turned out pretty smooth. I didn't end up using it, but it's worth a shot before replacing the links. And since they're of fixed length, you don't need an alignment unless you replace with an adjustable unit.

 

Thanks for the responses!

I'm not sure how the camber link (being a spherical bushing) could make a groaning noise. However, I can't say I've diagnosed many spherical bushings. I did one of the links off (the side that's groaning) and noticed the grease was dried up and the rubber seal didn't follow the bushing at all. It didn't seem seized though. I think I'll try re-greasing for now, or simply swapping the links to determine if the sound travels.

At the very least I could replace them with Ronin adjustable links, as I have ~2.7 degrees of negative camber in the rear that I'd like to take out. The alignment doesn't concern me, as my shop said they'd modify my alignment specs for free after my last one.

My worst fear would be the rear bearing wasn't installed correctly and is rotating in the hub from a start. I really don't want to source a new rear trailing arm due to a potentially scored bore.

 

mine makes groaning like yours, but in reverse. i think mine is an old caliper on a car that doesn't get driven much.

one other thought, it could be a bent hub, on a 1st gen when you tap a curb (they all have), it sometimes bends the axle, and it makes the caliper go back and forth, and this does make odd sounds. pull the wheel and see if the hub is straight

 

I guess it's possible for the hub to be bent. I did take the hub out to install wheel bearings/longer wheel studs when the accident happened. Oddly, everything fit back together without issue and I'd assume a bent hub would make for a difficult install. Alas......when will the rear end drama stop

 

So I swapped the rear vertical dog bone links from side to side. No change, as it still sounds like the noise is coming from the passenger side. I did find those spherical bushings/pillowballs inside the links are pretty much seized though.

I also noticed on my passenger rear adjustable toe rod, the spherical/pillowball portion (whatever it is) was cocked and not centered. So I centered that and re-tightened it.

 

It's difficult to troubleshoot irregular car noises just based on a vague verbal description. When my control links last went out, the car would make a squeaking/groaning noise every time the chassis moved. It was particularly annoying when driving at slow speeds such as in parking lots or over pothole laden streets. Are you able to record the noise on your cell phone and attach the audio file for us to hear?

 

I honestly don't think a phone or go pro will pick up the sound. I'm going to do some more investigating today.

Part of me is thinking the diff might finally be wearing out. It feels like the passenger rear wheel is slipping when taking sharp right corners (in addition to the sound I mentioned). Aside from my fluid change 10k ago, it's likely the diff has never been serviced.

 

Following up on this, I took a pry bar to all areas of the rear and found no flex that could cause the noise. It did seem like the caliper was still sticking slightly though, so I tore it apart and completely rebuilt it with a mazda rebuild kit. (apparently passenger rear brake calipers for 5 lug FC's have been discontinued by every single reman company and you can't buy them)

There was only a slight bit of corrosion in the bore and seal surfaces of the caliper, so I gently cleaned it and put everything back together. ALAS, THE NOISE STILL EXISTS. I'm starting to think its diff groan from bad friction discs.