Ok, another update.
Firstly a Handheld in the cabin will not pick up the noise at all! You cannot home in on it because it is not there, even with the squelch off.
If you open the squelch on either radio the noise is unchanged, but it is still only there at a certain RPM and when pointed at a tuned station.
If you change to an unused frequency and turn off the squelch there is none of the noise.
The noise does not appear on either NAV1 or NAV2 which are both VOR recievers, it is restricted to the 2 radios. It also does not appear on the ADF audio.
The 2 radios have seperate cables and antennaes and they both exibit the same behaviour, with the newer radio with the new antenna being worse ONLY since the antenna was replaced with a higher gain unit.
NAV1 is the second half of COM2 which is a KX175BTSO
All last week the issue was absent, the comms were perfect. On Saturday half the flights were perfect and the other half had a little noise. On Sunday it got worse and we stopped flying at mid day and cancelled the remaining flights as it was becoming a safety issue with stations in front of the aircraft becoming unreadable at times.
This morning the noise is there, it starts at 2450RPM and stops at 2550RPM on the ground, it is still directional only on recieve on both comms.
It is possible to park the aircraft close enough to the tower that there is clear line of sight between the antennae on the top of the aircraft and the tower aerial on the tower, even in this situation the behaviour is unchanged, clean audio until the nose if pointed at the tower then with the nose within 20 degrees of the tower the signal degrades to 2s and then back to 5s again in every other direction.
I'm still going crazy trying to find a solution! I appreciate the suggestions.