As I recall, I was going over one of the mounds near the fence in back and it died, and when it started again, that's how it sounded. I turned it over and didn't see anything - I didn't notice the blades were loose 'cause I didn't check that.
I suppose it *could* be as simple as the blade being banged up so badly it's out of kilter and working itself loose?
It didn't look dented, but it might have a big wow in it so it no longer sits flat.