Today I was mowing when my toro starting making a slapping sound from the deck. I took off the belt cover and discovered 'rubber dust' on the deck from the blade belt. The belt was what was slapping around....on what I do not know. I tried to put more tension on the belt, it made the slapping sound disappear. But still the belt seemed looser than it should have been. A couple yards later the belt come off of the pulleys. I put it back on, but soon after it fell off again. I only had a couple yards left and just used my 36" on them all. Apparently Im going to need a new belt for the deck. The mower has about 2000 hours on it....I can only assume it is the original belt...I bought the mower used. CAN BELTS STRETCH? Ive never had one stretch before? Do you think this was the cause of why the belt keep fying off? PLEASE HELP!