High pitch noise

I just noticed recently that a high pitch pulsating noise is being emitted through my speakers when I switch my pre-amp to HT bypass. Any ideas? My hunch it is probably the cable box, but wanted to get some ideas/suggestions before I unhook everything.

To give you an idea of my setup:

- NAD L/R pre-out going into my Placette HT-byass
Placette to Threshold amp
- NAD Center/Rear pre-out to Adcom

The amp, transport, and dac are plugged into a Richard Gray. The tv, cable, sub, dvd, Placette, and Adcom and plugged into a Monster powerstrip. The Monster power stip is connected to the 4th outlet on the Richard Gray.

Thanks,
Ryan.