If it is Raptor 30 you have, it has got the Wah wah decease.
It is the oneway bearing cuts in and out during descents.
The easiest way to get rid of wah wah is to install the "constant drive gear"(PV 00379) that is included in the Raptor 50 V2.
Could it also be that the tail rotor drive belt is too tight? I notice a surging, strange noise when I quickly chop the collective, but only since I removed and replaced the tail boom/assembly. The belt seems fairly tight, and when the main rotor is spooling down after engine shutdown, the tail rotor rotates
a bit.
I am going to try and move the tail boom in a small amount this afternoon and see if it alleviates the problem.