NEED HELP: AC Makes Recording PC "Pop", Do I need Power Conditioning???

I've installed a desktop PC in our sound room at church to digitally record our services and performances. It interfaces with our large Crate 16-port mixing board using a Lexicon Alpha Desktop Recording Studio USB module. The Lexicon connects to the board with a Radio Shake double RCA cable with a built-in line filter. Board and PC are both on power outlets from the same circuit and are well grounded. The ground on the HVAC units has also be recently checked by an electrician. The HVAC units are also on a different circuit and breaker than the sound equipment but the whole church comes out of the same breaker box.

With the PC hooked to the system, there is a loud "pop" when one of our two house HVAC units comes on. Also, a small amount of background noise is created in the recording when a HVAC system is operating. There is no pop when the PC is off and the problem did not occur until the PC was installed.

If you know about this kind of thing, please let me know what might be going on and what is the best way to fix this problem? Do I need to run another circuit from the box? Do I need a power conditioner, or what? Please help.