Thanks for the great replies. The ICOM radio's squelch is a part of
the volume control. You pull it out and hear the squelch and turning
the knob just adjusts the volume of it. If you push it in the squelch
noise stops. Doen't seem to me that the squelch is really controllable
except to change the volume of it, which also changes the radio volume.

I never knew about a squelch and gain controls on the back of the
radio. I don't have the manual with me. I'm now at a remote airstrip
where there is hardly any radio traffic, so I'll have to get someone to
transmit while I try it out.

I called Aircraft Spruces radio shop and got a little feedback from a
tech. He said to first check out my spark plug connections. He said
loosen them one by one and then retighten them. He said that's a
common cause of noise. And now that I think of it, I had the CHT wire
break and to fix it, one spark plug had to be removed, so I better
start with that one. I guess I could actually disconnect that one
plug and run the engine to see if the noise disappears. If that
doesn't work, he recommends a new ignition harness, which isn't too
painfully expensive. Thanks again for the help, especially forgetting
about using an external capacitor. Is that what those mag filters
basically are?