My car (190E 2.3 Avantgarde), has a slight whine in the differential when letting off the pedal and then just touching it. All minor parts inside has been changed, but the sound is more or less still there. Now a new diff or a used one costs a lot, so I just wondered if there is something more I could do? Is there a special good oil or something that would help? Please help, the sound drives me nuts!!!