Audio Systems: High pitch whistle from home theater speakers, home theater speakers, whistle sound

Question
Hello,

I am using a Sherwood RVD-6090R (60 Watts x 5 Channels) receiver with five 20-30 watt speakers attached. I get a constant high pitched whistle sound that doesn't go away when I switch audio devices with the audio selector. The sound is very subtle and always in the background. This sound happens even when there are no RCA lines going in to the receiver. Sound also increases with volume.
When I use the digital channel this doesn't happen.

Rich
Do you know what would cause this type of background noise?

Answer
Is it coming from all the speakers? Is it grounded properly?
I think you may have to call Sherwood about this. What is stumping me is why it would only be through the analog side of the receiver... I'm leaning towards a grounding issue, but without actually hearing it, it's tough to diagnose over the internet...