RF problems in the shack :(

Well eventually it appears when you least expect it, its the dreaded RF appearing on your phone transmissions, I know its not morse code but sometimes you just have to get the microphone out and actually speak to someone, it was during one of the RSGB 80 meter contests where I got the dreaded comment " you have distortion on your audio". Oh no you think to yourself , here we go again undoing connectors redoing connections this is not going to take 5 minutes to fix. So after a few tests we find any transmission over about 15 watts causes the RF to appear on my audio but only when I use my low half sized coil loaded 80 meter dipole. On my inverted L for top band, 80 and 40 meters there`s no problems with RF so thats what I use for the rest of the contest but any one with HF aerials will tell you a resonant half wave dipole (even a short one ) will hear better and transmit better. So today we start to trace the problem..first check is at the coax to aerial centre connection, so I drop the aerial down (I have it supported at the centre and both ends) and begin to pick away at the waterproof layer over the connections hoping to find a loose connection or water ingress. Connections are still solid and no signs of water, waterproofing is achieved by using a layer of Scotch VM tape(which is thick and gooey) and then covered with a layer of Scotch 88 electrical tape (which is slightly self amalgamating and will stick when its wet !). Anyway I remake the connections and check back at the radio..still the same..Grr! Next I improve on the choke balun I have at the centre, here I already have some tubular ferrites which I have fed the coax through so let's improve this a bit by making an air wound coax coil below the ferrites as well, I don't have enough coax to wind a proper choke so I wind as may turns as I can below the ferrites which should at least give a slight improvement. Back at radio for a check..he haw..nothing, not even a slight improvement..Grr! Now its raining so any other thoughts are put on hold for the moment, really can't complain it's the first rain for about a week. So as usual rain stops play , now what else could it be ?