Diagnose this BMW 635 diff problem?
 

I don't know why my brother keeps throwing his money away on this car, he could have bought a decent 911 for what he's spent to keep it running. I think it's an '87 635 CSi and he recently added a "custom performance diff" which started making "loud noises" within five blocks of the mechanic's garage. Would you say this was an installation problem or a product defect? My bro thanks you for the advice:

http://forums.pelicanparts.com/uploa...1151470320.jpg
http://forums.pelicanparts.com/uploa...1151470335.jpg

The only custom diff for that car would be a limited-slip unit. Did they fill with the appropriate fluid?

I know you own a shark, so I might be wrong, but I thought it was common to 'upgrade' to a 3-series diff (325ix?) to get different gearing.

E

There are different ratios (a few) but also posi unit or not. Are the gears making noise??