Hi chaps, another radio amateur here to just chip in a bit, for what it's worth. The directionality seems to me to be related to the signal strength of the ground station at that particular heading, compared with the on-board noise generator. Weaker in my opinion. OP, have you listened with the squelch wide open on the frequency(ies?) of interest when the ground station ceases transmitting? Do you hear the white noise now? Do you hear it on channels one up and one down with the squelch open? Trying to ascertain this would give us all a clue and prove my signal-to-noise ratio. If the noise is still present it certainly would appear to be on-board as there is a slight doubt in my mind that it is for sure. If it is, I strongly suspect a voltage regulator somewhere and after doing the usual load shedding try switching off each VHF one at a time in case one is affecting the other. I do hope the above can be of some use and hope you find and publish the cause soon! Good luck!