Popping noises coming from my HTPC

I have a home theater with a BENQ W6000 (1080p projector). Its only 1 year old so I don't plan on upgrading anytime soon.

I currently have the following HTPC setup for the home theater:
AMD X3 720 BE (unlocked to X4)
4GB RAM
2x Sapphire AMD Radeon 4850s crossfired (I believe they are each 512 mb memory)
Windows 7
Corsair 650W PSU


One of the most annoying problems that I have is that I cannot leave the HTPC on. If the HTPC is already on when I turn on the projector I get "popping" noises (almost like fireworks). I am guessing that maybe it has something to do with the 4850 cards because that doesn't happen on any other A/V device that I have. And it happens with any A/V receiver. I used to have a Pioneer A/V receiver and I got the exact same popping sounds as soon as I would switch over to the HTPC. I always have to turn on the projector first and then the PC to avoid getting the popping noise.

Anyone?

Speakers. I run the HTPC through an A/V receiver that is hooked up to my in-wall speakers for my home theater.

FYI. I just updated to an Nvidia 660 GTX. Problem persists.

Don't think so? What is reclock? I am running windows 7 x64 ultimate with very few additional apps. Never heard of reclock.

Do you mean O/S?? Windows 7 x64

Otherwise, I don't have too many apps. Just games and codecs (standard video decoding such as mkv, divx, etc)

Windows media player.

Note that the problem occurs when the HTPC is simply "on". This is not an issue that is specific to an application running.

If the PC is turned on (or already on) before the projector is turned on, the result is popping sounds from the speakers. If I turn on the PC AFTER the projector is already on, no issue.

I don't think we have the same problem. My problem is independent of a movie actually playing. It is related to whether the PC is "on" or "off" when the projector is turned on.

What do you mean by ground loop? Keep in mind that any other device that is always "on" does not have this problem. For example, my cable set top box is always "on". No problem. I have left my Bluray player and my WD Live devices "on" and there is no problem if I turn on my projector later. But for some reason, the PC results in this popping noise. So how would I correct this?

HDMI? I will try a different cable.

So I tried 2 different HDMI cables. Then I opened a brand new cable. Problem persists.

Interestingly, when I shut off the projector, the popping sound continues. I have to shut off the A/V receiver for the popping sound to stop. Once I turn the A/V receiver back on, popping continues. So the only way to stop the noise is to either (1) shut off the receiver, (2) switch to a different device on the A/V receiver, or (3) make sure that the projector is turned on before the PC is turned on. And once the popping starts, it can't be stopped without (1) shutting down the A/V receiver, (2) switching to a different device, or (3) shutting off the PC.

Note that I now have a brand new EVGA GTX 660 with HDMI connector for my HTPC.

Not playing a file. Just to be clear again, the status of the HTPC being "on" is the sole source of the problem. Pretend that I turn on the PC first and then turn on the projector 10 minutes later. The noises start almost immediately. I haven't even logged into windows.

Or if I leave the PC on (no apps running) and come back a day later to turn on the projector, the noises start and the only way to resolve is to reboot.