Differential Diagnosis?
 

I searched and am 70% sure I know what's wrong, but the threads I looked at either didn't have the same symptoms or were unresolved.

My car is an '82 S (drum brakes).

After I recently changed out my tranny mount and got the high flow cat from rx7.com, the car's been running A LOT quieter and revving smoother than before. I purchased an LSD third member a while back, and finally got around to having it installed. The diff didn't come with a pinion flange, so it was taken off my car and installed, the backlash was reset, etc. The place I took it to has been working with Mazda RXs for a while, so while I'm not discounting that could be the problem, I'm pretty sure it's not.

Now to the symptoms, this only happened right after the diff was replaced. Below 30mph everything's fine. Faster than that, there's this whining/whirring sound and there's no vibration associated with it. It's not too loud, but it is louder than the engine. It varies depending on engine speed, but it's not constant; sometimes it's louder when I go faster, sometimes not. When going through turns it will get quieter or completely go away, then when straightening out it comes right back.

This thread makes me think it's pinion bearings., but in other threads some say it's the actual pinion and ring gear.

So, am I on the right track ? And what are the differing symptoms from the diff bearings and ring&pinion?

Thanks. btw i love this forum.

Does the sound change when coasting in gear compared to under load?

Yes. The whirring is interrupted, and depending on speed, stays the same or fades away. If it's in neutral, it will go away.

Scratch the neutral part. it did the opposite this afternoon.

Could it be a transmission noise?

I don't think so. The only component that was changed was the differential, and the noises came out only immediately after installation, and I only here it back there. The car was running quietly before then.

but I really want to know what kind of noises a bad ring&pinion make as opposed to bearings in the diff.

What kind of fluid did they fill the rear with?

I don't know, but I thought that was the problem, so I put in Royal Purple + GM LSD Additive.

With your parking brake on and the tranny in neutral, if you rotate the pinion shaft by hand, how much free play does it seem to have? Does it sound clunky? Does it seem tight?

parking brake on and in neutral, the pinion is solid, doesn't move vertically or horizontally. It can only rotate a couple milimeters until I get a solid *thunk*.

with the rear jacked up, rotate the wheel on one side then the other side, also shake wheels, looking for bearing noises, just eliminating as we go here.

Check the park brake is completely off

I'll check tomorrow as well as taking it back to that shop (they're closed on the weekends).

When I shake the wheels, do i do it like the front wheels for bearing noises? Pull & Push, up and down, etc. ?

ZOMBIE THREAD!

Just for closure in case this comes up in someone's search, it was the pinion bearings. They made a high whining/whirring that stopped/started in when i changed a gear or let off the gas. There was a little vibration, but not as much as with a bad tranny mount.

Now the car is so quiet I can hear the other rattles and have been able to put more crap on my buy list.