Harman Kardon AVR 151 and AVR 171 SMPS buzzes for a few seconds when switched on

Hello!

I have two H/K home theater amplifiers. I bought the AVR 151 as faulty, the owner said it wouldn't turn on and the power LED was blinking. When it arrived with shipping though, it was already working perfectly fine. Probably this good operation is only temporary, it will not last long, because when switched on there is a not very loud but audible buzzing sound from the switching power supply section for a few seconds, which then stops, and currently all functions of the amplifier work fine.
Is there anyone here who is familiar with SMPSs and could tell me what this buzzing sound could be? Because it doesn't seem normal and could indicate some kind of fault that will come out sooner or later in the unit.
And a friend of mine gave me an AVR 171 which behaves the very same way, the buzzing sound comes for a few seconds, goes away, but all the functions of the amp are currently working fine.
So it occurred to me that it's not a fault, but it could be that the power supplies in both amps are dying.

Istvan