replaced the rear diff oil before i put the car on the road. stuff i took out seemed pretty decent to be fair, but replaced it anyway. ran the car for a few days and had a noise from what appears to be the diff. drained the oil out to fill with the recommended 80/90 oil and noticed the 75-90 that was in there had a load of little bubbles in it as though it was all aerated. rechecked the oil today and the same has happened again, i have done literally 10 miles since the change. the oil is at the right level, what could be the problem?