John Deere 757 737 Parts I have seen this happen and fixed it.
get under the deck, jack it up put jack stands under it. Take a sharpie and mark where the blade is closest to the deck. then spin that same blade until it gets to the same spot. and then mark it and do that for all three blades until you see one that is different. Take that spindle off, clean it up and you will see a crack in the housing. I can take a pics of mine tomorrow so you can see.