If I park 200M away from the Tower the problem is unchanged.
This is interesting because the VHF antenna has a clear line of sight to the VHF antenna on top of the tower.
At this range I can taxi in circles with the engine RPMs high enough for the problem to be there and the noise starts at +-20 degrees of the nose pointing towards the tower. The rest of the 320 degree rotation the noise is not there at all. When the noise starts coms go from 5s (everywhere else) to 2s for those 20 degrees.
I initially felt the at the signal was going "getting chopped" as it went through the prop, however I can put the aircraft in positions where there is an absolutely clear LOS between the transmitting and receiving antennae and it still happens if the station is aligned with the nose of the aircraft.
One of the major problems I have is that I am not a LAME, as this is a Charter category aircraft I am not free to take things apart myself, I need to bring ideas to the engineers or avionics guys who then charge me a fortune to see if they can fix the problem.
At the risk of sounding unkind, it does seem to me that in most cases today the standard procedure is to replace boxes until the problem goes away, often the problem is determined to be "the box" because a new one fixed it (sometimes because they broke the old one removing it). I am at the point now where they have replaced pretty much all the replaceable components in the system and I still have the issue and seem to be running out of ideas.
There also seems to be a lot of contrary theory amongst the techs.
There are 2 competing companies that have both had a go.
One says that the problem must be shadowing of the antenna by the fuselage in front which would cause a weaker signal and hence the directional nature of the problem.
The other says that the entire aircraft acts as an antenna and when the aircraft is aligned with the station you get the strongest signal and hence the directional nature of the problem.
Does anyone actually know which one of them is correct?