Parasound DAC 1500 soft crackling noise

[Salar]

Hi,
when I turn up the volume of my amp, I hear a soft crackling noise on the right channel of my Parasound DAC 1500.
It is audible with muted output as well when playing silence from any digital source.
Remember good old days, when the heads of tape decks wore out?
Sounds almost the same. like somebody recorded rain but heavily damped the trebles.
It´s not loud, but it is there and completely missing on the left channel.
Any idea? Bad caps maybe?
You can find the schematics here
To my ears still a great sounding DAC...
All the best,
Sal