High pitch noise at certain throttle positions

[kcriqui]

I've been reading the forum for a while now and gotten some really great information. I've come across a weird problem that I've not been able to find a solution for:

I just swapped the OEM amplifier for a 6 channel amp so I could control the level of the door speakers independent of the dash speakers in my 2001 Boxster S. It's wired up and working, but there is a high pitch noise when the ignition is on, whether the engine is running or not. It varies in loudness as I move the throttle. At some throttle positions, the noise goes away completely. It's always the same frequency and still there when the engine is not running so it's not ignition noise. I'm at a loss why the throttle position sensor could cause noise in the amplifier. Anyone else ever experienced this?