Is this a mechanical buzzing sound coming from the amp or is the buzz coming through the speakers? If its a mechanical sound from he amp itself it may be the transformer. I once had an amp many years ago where the transformer buzzed- the transformer was delaminating.
Jameswei is right, Pass does have a good service department. One of my 0's burst into flames one day when I turned it on. When they gave it back to me, it was perfect. Never caught on fire again, either.