I have a 2001 CLS with a AVIC-D1 head unit in it. Stock speakers and an amp in the trunk for 2 subs.

The subs in the trunk run off the RCA outputs in the D1, however the rest of the speakers run off the other output connectors the D1 has in the back.

Now the problem is that I hear a "Fsssssssssssssss" sound constantly at low volume. (anything below Vol. 5) If I crank it up, then it covers it. Going to Vol. 0 also doesn't stop it, or get it any quieter. I thought it was maybe the setting for the voice direction of the navigation. So i went in settings and turned that off, and it still buzzes. Also, this happens with engine on or off (not alternator related). Obviously with the ignition setting to "off" then the unit shuts down and the buzzing completly stops.

Any advice is welcome.
Thanks!