For about a month now I've been hearing a new sound coming from the rear end. It's a rumbling/grumbling kind of noise (not a sharp metallic clanking, or thumping) that oscillates in time with the speed of the wheels. It gets slightly more pronounced during sweeping right turns and tends to go away under heavy load (that, or it's drowned out by the exhaust noises), but it's always there. I can't reproduce it with the wheels off the ground. The wheel bearings seem tight, no wobble either vertically, or horizontally and they move freely with no stiction or grinding. The only thing that seems to be a clue is that there is (what I think to be) a lot of rotational lash . I haven't measured it precisely but, when off the ground, I can rotate one wheel approximately 10 degrees before the other one moves. I can't see any play between the hub-end of the axle shafts and the diff so I don't suspect that the CV joints are bad...but who knows?

BTW - this is on a '99 NB1 "10AE" with the original-to-the-car Torsen LSD that has approximately 330K miles on it. I change the fluid in it about every 3 years (the last time was last year) and - other than that - it's never been serviced.

One thing...there are two parts of the car that I've NEVER TOUCHED; the diff and the trans. That's because their internals seem fiddly and mysterious to me (sounds weird coming from a mechanical engineer, huh?). I guess what I'm looking for is some input on whether to;
1. Suspect the "cheap" parts first (hub bearings and axle shafts) and replace them
2. Replace the pumpkin and hope for the best
3. "Man up" and break open the diff to figure out what's going on and learn something in the process
4. Go "Full Ballerz" and replace the puny Miata diff with a Getrag, pat myself on the back and beg for street cred

 

I know next to nothing about troubleshooting the dif, but what you describe is classic CV joint failure noise. I'd double check those first.

 

Is that the mileage of the whole rear end? My first guess would be bearings - wheel or diff.

I haven't had a driveshaft failure (CV) other than on my OTM, and from memory that involved some clanking or similar noises. If you can get the rear up on stands, idling in gear, a stethoscope should be able to isolate bearing noise in the diff or wheel bearings..

If it doesn't seem like bearings, pull the driveshafts out and have a shop check them - pretty safe bet that it is one or the other (bearings or driveshaft failure).

 

No noise off ground.
Increase in noise when turning one direction, right
My money would be on left rear wheel bearing...
I would also expect the noise to decrease in left turns.

330K is a bunch of miles on anything, however.

All diff bearing noises I've encountered make some noise when up in air.

 

So far I'm hearing bearings, or axle shaft problems. Replacing the axle shafts is painful enough. Tackling the pressed-in bearings is NOT something I'm looking forward to.

 

So in my search last night - to assemble the necessary parts to dive into a rear wheel bearing installation - I came across this "Custom tailored rear wheel bearing assembly" from Planet Miata. They have a Facebook video up which shows that this is a complete assembly including the upright, new bearing and new hub w/ ARP studs for both sides all for $500! This may not be Miatahub-good, but it sure is the "Easy Button" I'm looking for.

Does anyone have any info on this setup?

 

Well, since no one responded I've ordered a set "for SCIENCE"!

 

Ain't they purrr-ty?