I not going to make any comments about your mechanic other than he doesn't know much about Eaton M90 blowers. The rattling sound is typically caused by a worn out/defective coupler. It
DOES NOT require replacement of the complete supercharger.
You can get an improved coupler
here to include other parts like snout seals, bearings and the GM supercharger oil. The work can be done DIY and alot cheaper.
The procedure to replace the coupler is in the prior link I posted.