I have a similar issue with a rear diff that makes "noise" which has been within BMW's parameters of back lash in the gearing. Now that I just excedded 100K miles I was told that even if I go 120K miles, because I made an issue of it 3x while under warranty, it WOULD be covered...so says the customer rep...
Still a proud owner of a now "older" "out of date" "non warrantied" BMW...