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.

Could not get your youtube post. Have you considered changing out to another wheel set to make sure its coming from the wheel?

Here's the vid:

Thanks Rolla.

I've tried another wheel and cassette which sound fine on bike. I have not yet switched this cassette to the other wheel since the noise is occurring when the cogs are stationary.

If the wheel bearings feel okay after being serviced, look for a new freehub body. Though I've never heard a noise like that besides something hitting the spokes.

Depending on its age, you may more easily find a hub and rob its parts.

Sounds like something rubbing the spokes to me also. It doesn't sound like any bearing noise or anything I'd expect from something internal to the hub or freehub.

Put the chain on a different cog just so you can eliminate the spokes hitting that briefly on their trip around the cassette. At least for us if not you.

You sure there isn't one spoke sticking out of the hub flange a little further? Though you'd probably have to remove the cassette to tell.

If the cassette wiggles "sideways" excessively, I think that would be a good clue for a bad FH body.

I was able to reproduce this with the wheel off the bike and skewer removed, just spinning it freehand.
Turns out it only occurs when the axle is in just the right position. Rotating the axle slowly (left hand) and holding the cassette steady, I was able to find a position where the noise occurs consistently (as in the video). Rotating the axle a bit further and the noise goes away.
Decided to take it into the shop and they guys there believe it is a bent axle. They are also somewhat stumped because the cassette is steady and spins like butter, but they are going to open it up tomorrow and operate.
Thanks for the help!

In case it helps others, the fix to this problem was to have my hub rebuilt. Local bike shop was great. After cleaning the hub failed to fix the problem, they didn't charge me any additional labor to repeat the effort and replace both sets of bearings. I've out a few hundred miles on it since the fix and it is spinning great and quiet again.

A bent axle does not rotate so if that was the problem the noise would have been consistent.
Probably a bad bearing. Good to know it is fixed. Don't use a pressure washer to clean your hubs! Keep water away from the seals of the hubs and lube periodically.

Mine did the exact same thing. LBS changed the freehub under warranty.