Mine (5-blades 22X22 diam/pitch) sounds like a box of rocks aft whenever I hit about 1400 RPM due to the too-close clearance of the prop tips and the hull in the tunnel, but at 2900-3000 RPM where I like to run the boat, the roar of the engine/exhaust is a steady roar - no noticed "rattle." Looseness of the rudder would be something that you would notice by grabbing it to see if it moves at all. Props can also be loose. Jump over the side and grab everything to see it it wiggles - tabs. strut, shaft, prop, and rudder. If it all feels stout, maybe it is cavitation. What do you know about the maintenance history of your prop?
FROLIC (2005 MainShip 30 Pilot II)
Panama City area