NAD 325 BEE a little hiss or buzz in power amp.

Hi there. I have tested all the supplies from the servicemanual. Everything is fine. It plays and all functions is OK. My problem is in the power-amp.

When no music and input on poweramp is set to GND at the PRE-out MAIN-in, there is a little hiss/buzz. It goes up and down. The left channel is very easy to hear,
the right channel is nearly silent but I can hear it very weak.

Its just like there is a "loose" connection somewhere. Just like an input that floats a little. But as said, plays music very good.

Does anyone have an idea I would be happy ?

BR Jens Hansen