I'm inclined to agree , RF getting in the radio . If you were TXing on SSB , the receiving op could probably hear the RF symptoms . Could be from leaky coax , or just proximity to the antenna . Coax is cheap , I'd be inclined to replace it with something better shielded . A good RF ground near where the cable comes thru the wall might fix you up , the splice point would be an easy place to try . What's your power supply ?