My NA Miata race car has a serious driveline whine, and has ever since I bought it 3 years ago. Problem is, I can't tell if it's the transmission or the diff (or possibly both). Now that the AutoX season is over and I'm working on the car again, are there any obvious signifiers that could help me differentiate the two? I changed the fluids in both before last season. They both had some amount of metal shavings in them. The diff seemed worse, but when I pumped more fluid into the trans at a later date, it did seem to quiet down a little bit, at least for a little while.
Acceleration? Deceleration? Both?
It's loudest when accelerating or on throttle. It's much quieter in deceleration, but it's still there, faintly. It gets louder and higher pitched the faster you go. It's not really noticeable in 1st or 2nd but it starts to get loud in 3rd and by 5th it's screaming.
Does it change with the clutch?
The clutch doesn't seem to affect it, as far as I can tell. If I'm coasting down to a stop, it whines whether or not the clutch is in. The whine is proportional to wheel speed, not engine speed.
Then you probably have a diff problem.
Take free advice for what it cost. 
I'll take your free advice... erm... freely 
. I had a feeling it was gonna be the diff, just because it's the more expensive item to replace lol. Thanks!
Ranger50 said:
Then you probably have a diff problem.
Theoretically it could also be related to the output shaft on the transmission, but I agree that diff is more likely.
Toyman!
MegaDork
11/9/22 12:20 p.m.
You can jack the car up (both rear wheels or the entire car on jackstands) and if you have a helper, have the helper run the car through the gears while you are underneath the car, you should be able to hear which assembly is making the noise.
If that doesn't work, then take the driveshaft out. Then run the car through gears, if there is no whine then it's the differential. If there is still a whine, then it's the transmission.
If the noise goes away in 4th it's transmission.
