It certainly sounds like something is getting in the signal path, be it interference from anything from long speaker cables picking up radio frequencies, to a power supply hum which could be from virtually any component (my computer monitor has a high pitched whine similar to the one on my mother's 27" RCA tv and a friend's 17" Flatron monitor...all from the power supplies) or a faulty DAC in the dvdp.
Switching out various components should help you to isolate the problem but don't forget to have a listen to anything that is plugged in, not just the dvdp and receiver.
