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.
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.
One way or another it needs attention. You need to decide if it's worth it.
If you're going to continue using it for a while, might be worth putting a voltmeter across each of the speaker outputs - check there's no more than a few
DC millivolts, to damage your speakers (though if it's a loud buzz, that won't be doing them any favours anyway).
A new (good value) amp might be a good idea.
45 years wasn't a bad life for the Sony (more than any new amp is likely to get).