The best way to check for local noise sources, is "divide and conquer". Start by shutting down EVERYTHING except the radio
power. If the noise is still there--it isn't local. If the noise has gone away, turn on 1/2 of your breakers. left/right, top/bottom, any way you want to split it, turn on half of them. If the noise isn't there, turn those off and turn on the other half.
As you keep testing "half of what's left" you will be able to quickly break it down to "this half" creates the noise, "that half" doesn't. And for the average boat, you won't have to divide your halves more than 4-5 times before you get it down to one or two circuits.
Electrical noise can be a RFPITA to track down, but it feels so good when you finally DO nail it down. Mainly, it needs patience, and assuming
nothing. A doorbell, an old transformer, a loose plug...noise is like rust, it finds the most peculiar ways to spread.