Welcome to MLMF. It does sound like you are on the right track. Having replaced all the belts, and an idler pulley, and confirming that the blade mandrels are free. Is the sound you say you hear something new that you didn't notice before?
Some PTO clutches can be adjusted but it looks like from the replacements I found for that model online it might be non adjustable. Maybe it has just worn to the point it can no longer lockup tight without slippage. One thing to check would be to make sure your getting a full 12Vdc to the clutch windings. A lower voltage due to corrosion of either the positive lead or ground connections might cause it to slip.....I am noticing a bit of a squeel with the deck removed, which seems to be coming from the PTO clutch itself. The PTO engages and disengages but it seems to be making a bit of a noise and I am wondering if it may be the source of my trouble. ...