[BigLew]

A poorly grounded motorized device; radome, pump motor, maybe. Possibly a poor connection on your depthfinder. Not "at" the connectors , but inside the connectors where the various leads are attached to their respective pins.

Did you assemble your VHF radio connector yourself or was it supplied already assembled? Either way possibly the outer coax shield is not well done. That can be tricky. If you antenna lead passes any motor or possibly a CRT display and has a poor connection of the shield (woven wire surrounding the inner core lead), you could be picking up some sort of RF signal being generated by some elctrical device onboard.

Two suggestions:
1. Try a different antenna, even if you have to borrow one for a couple minutes from a friend.
2. Get on a clear channel and have a friend transmit to you a slow count while you try turning off and on other electrical devices to see if one of them is generating the RF.
3. To keep in mind that the source is more than likely closer than farther way from the radio connections. That's not to say it can't possibly be at the other end of the boat.
4. If you have a friend who's a ham radio operator, asking if he might take a look at or have any suggestions.
5. As a last resort, or probably your best option is to get in touch with your marine electronics service person who has a great reputation.