Recently when I am mowing, there is a strange sound coming from the deck. Kind of a grinding sound or as something is vibrating. It does not happen often, but it happens on and off while cutting. It is only when the blades are spinning. My ideas: 1) Shear pins are loose? (does this happen), 2) Blades are unbalanced. It does not sound like a gear box issue, it sounds like the blades are chattering, but not hitting one another. Fluids are fine, everything gets greased either on Saturday's or Sunday's of every week. Mower has ~70 hours on it. I am going to put on a new set of blades, cups and shear bolts in the morning before I cut, to see if that solves the problem.