Thanks for the replies so far.
After a bit more investigating I can offer a bit more detail.
The problem is more distortion than noise.
I tried several frequencies on the ground this morning and the lower down the frequency range I go the more distorted the sound becomes when I push PTT and the more noise too.
Hasherucf:
I use a bird antenna analyser which gives a great graph of VSWR tests.
I think you're on the right track with that and will try to find someone with that equipment.