Please help Macallister mower making strange screaching sound?

I have a Macallister 1700w rotary mower (mac1700rma) heard a bang from inside. When i stopped the mower as the blade stopped the mower made a screaching sound opening up the motor compartment i found broken bits of what looked like the disk used in cley pigeon shooting but when the bits are put together there is a square hole in the middle. The mower works but makes this screaching sound when the blade and motor are slowing down to stop. Blade and belt are fine.