Class D noises what gives?

To start off, this is not alternator noise.
On one vehicle, I have a 5 channel Soundstream Tarantula class D. When it starts up, there is a strange high frequency digital static only on the rear channels. It persists no matter the volume setting until I play something (music, voices) and then it goes away immediately and stays away. I can live with this.

On my other vehicle, I had a 5 channel Infinity Kappa Five. I had no noise on this. I swapped it out for a Kappa One and a Kappa Four to double the output. At first everything is fine, but if I drive them at a loud volume the rear channels on the Kappa Four will get a similar digital static that does not go away no matter if the engine is on nor off or whatever the head unit volume is. I've tried playing with the crossover switches and gains as well as the input switches, no difference. It is normally set to full range with all filtering done at the Alpine head unit. Unplugging the RCA cables makes no difference so I'm sure this is being generated inside the amplifier.
Speakers are all Infinity Kappa but there was no noise when using the 5 channel, just not enough juice to push the 3 10s.
Front and rears are rated at 2 ohms, subs are wired to 2.33 ohms.

This is driving me nuts. Is there anything I can do to fix this?