Subaru Repair: DIFFERENTIAL NOISE, Subaru expert help

Question
    Ive been working on a 1998 subaru legacy outback awd.
cust complained about noise in rear differential.Ive put
3 used differentials in,but there is still noise coming from the rear differential.It comes and go's.But it is much
better than the original one.Can you help me solve this
headache?

Answer
You didn't mention whether or not the car was a standard or auto transmission.

Are you positive that the noise isn't reverberating through the drive shaft and the noise may actually be coming front the rear of the transmission.

Also if you put a rear differential in with the wrong gear ratio it will make noise.