Would this be the main drive belt on the deck, or the one that goes from the pto on the tractor to the deck?
I would confirm that all of the hardware on the deck is securely mounted. Any loose pulleys, tensioners etc could cause this. I would also confirm the pulleys are not wobbling on the deck due to a cracked or rust eaten base. Not saying there is, but sometimes when there is a bunch of grass build up in and around the bases, you don't even know this is happening.
If it is the belt that connects to the tractor itself, I would confirm the pto pulley/or unit itself is not loose.
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