Having had a close look at the suggestions previously offered, I suspect that the "white noise" is just that. The problem probably relates to a diode or zener diode being triggered into white noise mode by a particular range of circumstances. Humidity playing a part, could also be indicative of some capacitive coupling that changes marginally and aids to trigger the problem.
Now you'll need a "portable" white noise detector to home in on the probable source.
The fact that reception is poor from on the nose, doesn't mean that the source of the interference is forward, in fact the maximum pickup of the noise will be on the side of the antenna, which gives you a huge scope. So, look closely at the airframe wiring schematics and identify the location of any possible culprits. They don't necessarily need to be diodes, a corroded connection could also exhibit similar symptoms.
Your problem, to reproduce the effect on the ground!