Revox PR99 buzz

I have a Revox PR99 that works well but sometimes I hear some quiet buzz at playback, like some really high 50Hz harmonics. I'd say that it's most noticeable around 6 kHz, but I can hear it pretty much in the whole upper spectrum. The interesting thing is that if I press stop and then play again the buzz is usually gone.
Does anyone have any idea what could be the cause?

Any help would be great, thanks