Monkey with a Football
Join Date: May 2005
Location: With VPN, anywhere
Model: Linux, Firewalls & VPNs
What you are hearing is amplifier output noise.
The intercom and the CB each have their own amplifiers and they are low quality mono amps as compared to the higher quality stereo audio amp for the FM and music system.
You won't be filtering it without adding your own low pass filter to each system, internally. And if you did you would lose clarity.
“...and the rabid dog they gave me
wasn't too much fun”