NA6 diff noise

[Sparra]

I have attempted to upload a sound file, in both M4A and MP3 formats. Both were rejected 'The extension MP3 (M4A) is not allowed'.
I was hoping someone had heard this noise before, and was able to fix, without replacing the diff, but until I can find an acceptable sound file format for this forum, I guess my description of sounds like a 'whistling noise' will have to do.
The car is a 1991 Eunos, imported by the owner from Japan. It has travelled, not counting the trip from the land of the rising sun, 166k, 30k of which are down to me. The diff oil was changed out at 150k using Penrite 75W90. The noise started three days ago, once the car has reached normal operating temp (about 4-5km). The diff is not hot, and the oil level when checked tonight is sitting just under the fill plug. When a rear wheel is turned, and the car is in neutral, its partner turns in the same direction, so I am assuming this is a VLSD.
I recorded the whistling noise after taking it for a drive to warm up, then placing it on stands and running in 4th gear at idle. The noise appears to be coming from the pivot (ie diff input shaft).
Any ideas please? And if someone can advise what sound file formats are permitted I can upload the 'real' sound.

[Sparra]

I have made a video of the noise and placed on youtube here - http://youtu.be/mZgtg5zyFJg

The rear calipers and discs have been removed for this video, to discount the possibility of brake rub.
The oil in both the diff and the gearbox was replaced for this video (Penrite 75W90 full synthetic). The replaced oil from the diff was fairly black (only 6k old) but I can't comment on whether it was indicative of MX5 diff oil.
A transmission specialist mechanic inspected the car yesterday and advised that he thinks the noise is gearbox related, but only 90% sure. He said that if it were up to him, he would pull the tailshaft and run the car to see if the noise continues without the diff being in the equation, then make a decision about what to do. As a gearbox rebuild would cost in the vicinity of 1500.00 through his workshop, I am loath to go down this route. Especially as I can purchase a 'good' secondhand box from Automotive+ for 440.00. Any feedback re this topic would be appreciated!

Cheers

[hks_kansei]

Both wheels turning together is normal for any diff when the car is in neutral since the crown turns and the tailshaft just spins. If you do it in gear (1ts or reverse) the tailshaft wont turn and you'll be better able to feel if there's an LSD or not.


Anyway, that has no bearing on the noise.

I can't listen to the sound at the moment (at work) but if it's coming from the diff area, specifically where the tailshaft goes in, it could be the uni joint (bendy joint in the tailshaft) or possibly the input bearings on the diff.

If it's the input bearings in the diff it's best to go to a shop, replacing them is an absolute arse of a job, and needs a press.

I've only pulled them out, and that was enough of a stuff around, let alone putting new ones back in (note, the bearing will probably be ruined in the removal process)

A uni joint needs the whole tailshaft replaced, as Mazda decided to be bastards and not use replacable universals in them

[Sparra]

Following the advice of the transmission specialist, I did to following on Friday afternoon.
Drove the car to warm everything up, and check that the noise was still evident. (yes)
Put the car on stands, position 800mm shop fan in front of radiator. Mark the tailshaft position to the diff mounting. Rotate wheels to check for noise again. (yes)
Disconnect the tailshaft from the diff. Check for noise by rotating wheels. (no noise)
Remove tailshaft from gearbox, after positioning drip tray. Cut 600ml coke bottle in half and zip tie over gearbox output shaft housing (to catch oil while running without tailshaft.). Turn on fan and run car at 2500rpm, in fourth gear, for a period of time, to ensure everything was warmed back up again. (no noise). Shift to neutral and turn off. (Dropped about a tablespoon full of oil into coke bottle)
Shrug and swear a lot, as the testing proved that there was nothing wrong with the diff or the gearbox. (this of course was a good thing financially, but meant that I still had no idea of what the noise is!)
Put everything back together again, ensuring correct alignment of tailshaft to diff.
Drove around today, including a trip of some 90km at highway speeds, and 10km or so of suburban travel.
NO NOISE TO BE HEARD!!!
Can only assume there was dust or debri causing the noise, which was cleared when the tailshaft was removed.