I've been having a nice click-free week. What I did was remove and clean the cassette again, this time oiling the cog carrier rivets before putting it back. I don't understand how play in the cog carriers could have caused these exact symptoms (namely the fact that during my off-the-bike tests, rotating the rear wheel backwards while the drivetrain was stationary made the noise go away, only to come back when the wheel had rotated some multiple of 360 degrees), but anyway, it seems to be gone for good.
Bought a Velocity wheel with generic looking hub last year and lately was getting the same noise exactly when and how you described. With the wheel off and cassette removed I can grab the freehub and when engaging by hand it will have a slight pop/click sound like it does not seat fully at first. It only does this every third time or so leading me to think it may be a bad cog that is skipping along before engaging. This is a sealed unit that attaches via the non-drive left side with a 12 mm long socket. My problem is I'm not sure what or where to order a replacement because I don't know what brand it is. I have found practically nothing on the web until I came to this post. I'm sure I need to call Velocity after the holidays but was hoping someone had some ideas.
Thanks.
Gene