Sounds silly, but back the car up- say about 100 feet. See if the sound goes away for awhile. If so you can count on it being the rear boots being shot and it needing a reboot/grease. You can always fill the damaged boot up with grease, wrap the whole thing in rubber (from an inner tube) and use a couple twist ties to hold it all together as a temp fix until you can get it fixed correctly. The repair itself is pretty easy. Easier if you have access to a lift of course. My local garage charges me 60 bucks to put the car on the lift and swap out the rear end/axels etc. That's with them doing the work and me supplying the parts. He did the "Temp" fix for me for free. I know I'm not the only person to do the temp fix, as the 85 TD I just bought has the exact same "fix" done to it...

Personally, I would just get a set of them from Vstech, know they where done right (he has the correct tools for the job) and then not have to worry about it for the rest of my life.... (which for the record is exactly what I'm planning on doing for this 85 TD I have as soon as I have any spare cash)