Never heard of it with that type. It's a good bit of kit, the 75.
Had a 42-300 with a similar problem of props not quite balanced properly which made the a/c shake it's tail occasionally. It was like the aircraft had a shiver up it's spine!
I take it it's your terrain which requires steep descents?
Have you tried Power Management to MCT to see if the frequency changes with the increased prop RPM?