We have a 2.6 SE A4, 1996 (right hand drive). After driving for about
20 minutes it develops a high pitched humming sound. To the driver it
sounds as if it's coming from the left, and to the passenger as if
it's coming from the right, so therefore we think it's in the middle.
It doesn't appear to happen when the clucth is depressed, and is most
noticeable as you start moving from a standstill, although it is
constant on the move. Any suggestions as to what it could be?