Tiltorotor's motors sound 'gravelly' during transition from FW to MC flight

I’ve been tinkering with the tiltrotor.cpp transition code, so I don’t expect anyone to have an exact answer to this as the problem is likely unique and brought on by my own incompetence, BUT:

Has anyone ever experienced their motors making a crazy grinding noise in specific flight modes? It’s like the ESCs are getting a noisy signal only in this one mode (TRANSITION_BACK) - and if fact, based on examining it with the scope, the phase of the signal seems to jump around when this behavior is occurring.

As for what I have changed, the position of the tilt mechanism is now determined by an auxiliary channel knob rather than on a timer. Do any other pieces of code besides tiltrotor.cpp/h affect behavior during a tiltrotor’s transition?

Any and all crazy ideas appreciated as I am totally stumped…

[Resolved - I created a stupid situation where moving the tilt mechanism from FW_MODE to an intermediate position set the flight mode to TRANSITION_BACK, but the next time the loop ran it would get set to FW_MODE again, and so on ad infinitem, creating a garbled output signal.]

Oh, also-- what determines what motors are affected by set_rear_motor_state()?