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.

I am having a similar problem. I have an Integra DTR-7 AV receiver connected to a Plinius 9200 integrated amp via the pre-outs. I noticed a loud hum coming from my speakers when watching movies with the digital output coming through the AVR. Someone told me this might be due to the integrated being grounded and the AVR not. This doesn't seem to be the case, as taking the grounding prong out of the equation does not make it better. When I run the integrated for two channel music, it works fine. I've tried every permutation, and it has to be the AVR...I was told that Integra has had similar problems.
In case someone is curious, I ended up running my cable box's coax through the Monster power strip. The built in filter took out most of the noise, but it is still faintly there. My next step is to replace the interconnect between my NAD and Placette.