I've started recently to notice my cassette wobbles from a little bit. It doesn't do it all the time. I will be spinning along, then notice a little clanking sound, look down, and the cassette is going left and right, not as a whole unit but the cogs will point towards the left side of the bike, then to the right. I'll change gears a few times and it goes away, spinning straight as it should. I had the wheel into my LBS and asked them to look at the rear wheel. They didn't see anything wrong or note anything. What could it be? I just put a new chain on and am waiting for the LBS to get a cassette in the range I want. It did this both with my old chain and now with my new chain. I am running an Ultegra double.