I have a Steiner 450, tension pulley hits frame when I engage PTO to run mower deck. Seems tight when not running or not engaged. As soon as I engage PTO the pulley starts hitting.
I'm sure you'll get much better replies since I've never heard of Steiner (I looked it up and they're super nice), but I'm thinking a belt tensioner, maybe belt has stretched?
I'd remove belt making sure pulleys and tensioner are ok, spin freely.
It probably has grease fittings on everything.