ST1 Knocking - Solution?

I have about 4000 miles on my ST1. I pedal hard in city mode @30% about 22-26 mph. Recently I developed some knocking sounds coming from the rear I believe. If I go up to Tour or Power mode the knocking is more frequent and louder. I can wobble the cassette a mm or two with my fingers, I don't know if this is by design, or something wrong. Anyway, without working on it in the cold garage, what is the back end made of, freewheel or hub? Is there a way I can fix this with a little grease, or do I bring it in to the LBS. Please advise friends. Thanks.
I have had the same on my ST1. The bearings in the cassette is worn out. There should be no wobble. Changing the cassette will solve it.
Bring the wheel to a bike shop. It takes only a minute for them to change the cassette.
I had same issue some time ago. I fixed it by tightening cassette with $5 tool from LBS.
LBS put on a new cassette and no more knock. They also put on a new chain. He said the chain checker dropped in easily and actually wiggled a bit. Whether or not that contributed to the problem is in the air, but it needed to be replaced after admittedly being poorly maintained for 1500 miles.