Is the clattering noise concurrent with PTO engaged - or disengaged - or both?
M/W, suggest you get full model number and serial number of tractor - go to Snapper website - try to download owner's manual and parts list. S/B detailed info in those docs to help answer your questions unless more info is forthcoming from other posters to this thread...

The CLUTCH friction surfaces are not "lined", metal to metal is how they are configured.
If the noise occurs when you ENGAGE the PTO clutch, you need to be checking the blade mandrels and spindle bearings condition and any idler pulley bearings before you spring for a new PTO clutch.
The bearing in the PTO clutch is active only when the PTO IS NOT ENGAGED. So a defective PTO bearing would exhibit noisy, rough operation BEFORE you engage the PTO and get quiet after you engage it (because when it is engaged, the drive clutch face, bearing, and driven clutch face all rotate together as one mass, so the bearing rollers or balls are not operating.
If the PTO clutch is the kind that has a built-in "blade stop brake", the above statement might not apply as a defect in the blade brake could make noise when the PTO IS ENGAGED.

Thanks, mownie for the input. The noise is when the PTO clutch IS NOT ENGAGED. What is the "blade stop brake", and how do I tell if that's what I have. Again, what is involved with changing out the clutch? Can a reasonably handy guy do it in his garage, or should I leave that to the guys at my repair center?
RCBE, I have the onwers manual/parts list, but it's no help, giving only the part number and no other info.

Blade brake is a feature required on modern mowers. Some OEMs incorporate the blade brake as a mechanism on the deck while others have gone to a PTO clutch with the blade brake built into that component.
Blade brakes or blade stops are supposed to stop the blades from rotating within a few seconds after the PTO or the deck is disengaged.
Ostensibly to prevent injury if a person reaches into the blade area to clear an obstruction without allowing enough time for the blades to stop spinning before reaching.
Because the noise is occurring when the PTO is off, I suspect a faulty clutch bearing.
On most older PTO clutches (without brakes) the clutch bearing could be replaced after removing a couple of snap retainer rings (clips).
Now, for about 50% of the clutches built, the bearings are not serviceable and the clutch assembly must be replaced if a bearing fails.
Only you can decide if you are handy enough to do the job of replacing the clutch.

"RCBE, I have the onwers manual/parts list, but it's no help, giving only the part number and no other info."

Possibly that part # might help you find out clutch mfgr (there are several such as Borg Warner, Oguro, etc.).
Then some web searching might find you details for if the bearing alone can be replaced.. or at least some less costly sources for a replacement clutch (they can be $$$). Good luck.