Pioneer Deck and a popping sound.

I own a Pioneer DEH-6400.  Everytime I lower the volume all the way down to 0 it generates a "pop" on the Left Rear channel both on the hi level and lo level outputs.  This also happens when i switch sources and use the Easy EQ button.  It is only one pop per button press not a constant popping.  It is very annoying and you can imagine what a simple pop sounds like when its run through 1200 watts of power.  I tested everything.  The patch cables, power connections etc.  and nothing.  My co-worker suggested it might be the internal power supply.  Are there any other suggestions?  Thanks in advance.

i would suspect an open ground on the audio path in the unit for that channelKevin Gerry

Certified Electronics Technician

MECP First Class Installer

Owner/Installer

Classic Car Audio

since 1979