Yeah, this one's common. You're probably damaging the coupler.
DIAGNOSIS: Going straight to high speed with chunky frozen items wears out the internal teeth.
FIX: Don't start on high. Use the pulse button first. Hold the control dial from (O) to (P) in short bursts to break up the big chunks. Once things are smaller, you can switch to a variable speed.
IF THAT DIDN'T WORK: If the noise is a constant, severe grinding even on low speeds with soft items, the coupler teeth might already be damaged. You'll need service.
HEADS UP: This method actually gets you a smoother smoothie faster because it lets the chunks fall to the blade instead of just floating around.