I agree with @erik_squires that is possibly a leaky cap and it should be looked at ASAP before DC damage to the unit or even speakers occurs. My experience has been progressive deterioration starting with some noticeable distortion to a pop during turning off and on, or intermittent during playing. You state it is an older unit. If it has never been recapped that is my best guest. You can try The Service Department in CT. They service many out of production high end electronics. However, your brand is not listed, they are pricey, and it may not be cost effective to use them. In addition, their turnaround time is long since their flight pattern is full. They do excellent work. I used them for recapping Krells before I traded them in. They use higher quality parts than Krell and there was some improvement in SQ after they serviced them
30 Unquowa Hill St.
Bridgeport CT.
06604-2149
203-331-0671
steven@theservicedepartmentCT.com