My Ultegra WH-6600 rear wheel is making a grinding noise when spinning. I don't hear it when pedaling.
I had the freewheel serviced to clean out the gunk but the noise is persisting.
Very noticeable in this video: <slash><slash>youtu.be<slash>IQkokmS1fAg (apologies, I cannot post links yet)

Nothing external to the hub and cassette are rubbing, so I presume it is the bearings or something in the hub.

Any ideas on what is causing this and what portion of the hub I should look to service or replace?

thank you ... I appreciate any help.