Well, I have my idle adjusted to 800 rpm and I can hear a slight rattle/knock in the blower, if I idle it to 1000 rpm the noise is gone. Is this normal or do I need to start looking for another blower?
I would think it's more likely the rotor bearings that are shot. In which case, you need to find another blower, or get a new rotor pack ($500 or so) or a Magnum Powers specially rebuilt one ($550 or so).
Rattling is usually from the spring loaded internal coupler. Not really a big concern and even the solid couplers will rattle a little when worn. Try closing the bypass valve with your finger while it's idling to put a little load on the blower, and see if the rattling stops.