You are probably suffering from "microphonics" which is your circuit board inside the audio unit acting as a microphone into your ICOM. Turn your intercom on and tap on your gas door or just above it toward the audio unit. If you hear that tapping in your headphones, you got it. It is more prevalent in '01 - '05 but you don't say what yours is.
Turn down volume of intercom to below 9 is the only solution from what I've been told from those more expert than I in using the ICOM. I rarely use it but when I do I use FUZE headphones which cancel out most external noise so I know it's coming from the audio unit.
I didn't have the intercom on, had the CB on. We were doing the ride for kids, and was honking at people waving along the route. Normally I'm not a real horn user either. I had turned the squelch down somewhat. We'll see. :shrug: :shrug: :shrug:
It's actually a harmonic frequency of the horn coming into the CB receive. The same thing happens if you are watching channel 2, 54 MHz, and key the CB, 27 MHz. The horns must be down about third or fourth harmonic. I know that I took the horns off my 02 and switched them with my Valkyrie and they interfered with cb on the 02 setting next to the Valkyrie. I tried to adjust the freq. but the loss in db was too much so I just did as the others suggested and set the squelch about 2 clicks higher than normal.
The horn has make/break contacts that is actually a spark-gap transmitter much like that which was first used to send Morse code in early radio communications. You get the same effect from dirty insulators arcing on power lines and interfering with you CB reception when you pass near them.