I was just looking at the manual on Scag's site for your machine. It looks like your spring is a fixed unit with no way to either tension, or loosen it like I can do with mine. I'd just change the belt out and try that before i got too worried about anything else. I bet it smooths back out. A worn belt can make it vibrate pretty badly, and it'll just get worse as the belt gets worse. eventually, you'll prolly end up on a property somewhere and have it snap. I KNOW you don't want that to happen.