What is it? ......what is it?
It's another one of those old amps with plenty of switch / control contacts - which often cause random problems. It's possible that your sypmtom is component related though.The amp is a Sony TA - 1150 , the buzz comes from the speakers and is temporary for around 20/30 seconds. It is a constant sound for this duration only. After 20/30 seconds it suddenly stops and the amp performs without any flaw whatsoever.