My antenna is in the attic, about 12' from the rig (above my head).
I have a ferrite split-bead on _every wire_ into and out of the rig and computer! Without those beads, RFI completely destroys my signal (and sometimes makes it difficult to stop transmitting!).
There's an easy test:
. . . Reduce the rig's output power to as low as possible with its "RF Power" knob, or menu setting.
If the problem disappears, and you can transmit normally, you've found the trouble -- it's RFI.
PS -- as StayVertical says, this is a well-worn, deep track!