First, I would
NOT run it again until you get it figured out. Too much risk of major damage. Don't start it to answer my questions below.
Are you sure the tractor and mower are compatible? Did they come to you as a pair? What models?
This the first time using the mower?
Did you notice any vibration?
Does the sound change if deck is raised or lowered?
Does the mower drive shaft move on/off the tractor mid-PTO?
Are the splines showing on the tractor side (meaning yoke isn't on very far), or are the splines showing inside the U-Joint (meaning it could be on too far)?
I'm with hagrid, more likely a spindle, belt or belt pulley. But good to start here, as a problem here can be a major expense to fix and can cause major damage in a short time.....
This is my BX2360 mid-PTO connection. You can just see some spline inside the U-joint. Again, not likely same tractor or mower.
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