Teki
Had a similar scenario to yours for a while, although it wasn't tied to TQ settings, just high speeds at lower density alts.
It was one of the banana seals for the flap tracks on the wing/fuse fairing, it was almost non existent. Airflow getting into the big box and buffeting around. The noise/vib was more pronounced mid cabin in this case.
Have also had those clear prop leading edge tapes being partially peeling off, causing a bit of fluttering/noise at certain speed/power combos. Nothing siignificant though.
Miserlou
My pet hates - the constant rudder trimming required, esp single engine, and that bloody alt select knob - why didn't they put a detent for every 100ft like most others do? And of course the poor mans APU