racer bill

Hey guys, has there been any common problems for diff noise? Its not a whine, its more of bad bearing sound. Its an automatic car with 3.38 ratio. I noticed that 750i have the same ratio. Are the diffs the same just different rear covers, or are they completely different? Found a lot of 650,750 and M5 diffs used. 550i for some reason are much harder to find used. I know there are several places that offer remans (diffsonline, Dinan and Turner), but uncle sam is asking for money as well and I trying to save a little money right now. One last question. If you change ratio's, is there any programming required? Thanks for all the help