I can tune both comm units into the same station in front of the aircraft and the one with the antenna on top of the aircraft gets the noise and the one with the antenna underneath the aircraft does not.
This fits with the prop (or something else towards front/top of plane) becoming unbonded as rpms increase.
To help visualize the possible prop issue think of one element on a yagii (tv style) antenna being rotated, as the elemnt comes in line with the others the gain will increase, at 90 degrres it will decrease.
Noite that it is possible that an ungrounded prop (or whatever) is actually increasing the antenna gain, any fast cycling of Rx strength could upset the AGC and cause the issue.
On the practical side will it work to simply relocate the other antenna or does the bottom location cause other problems?