I hit a rock with mower now getting grinding noise, checked belts and blades but seems normal. Engaged PTO and engine stalls,spindle not loose but blade will turn by hand.
I wonder if the blade is cracked? Can you hear the noise when you hand turn the blade, to get a better idea where it is coming from? If not, I'd replace the blade and see if that changes the noise.
I've hit a flower garden rock and a fire wood stick both times it ended up being a blade issue. New blades fixed it.
Good luck.