grinding from rear hub
 

i have a grinding from my rear wheel when it it spinning free... i do not hear it when the cassette is spinning with the wheel (turning cranks with little/no tension)... is there a bearing under the cassette that this could be? the guy at the LBS said it has to be tossed and replaced, but i can't remember the part name... cassette is shimano 9 speed

i am thinking i need a chain whip and a special socket?