Onkyo TX-DS838 AV-Receiver noise on loudspeaker after warming

Hi there,
I have an Onkyo Receiver TX-DS838.
If the device is in operation for a long time and gets warm, I have a scratchy noise on the loudspeaker outputs A and B on the left that gets stronger and stronger, regardless of whether a signal is present or not. I have already tried to narrow down the circuit board based on circuit diagrams, my guess is due to the amplifier circuit board, but at least nothing reacts to cold spray etc.
Does anyone have any idea what that might be??
Thanks!

Best regards

McBerry