I would definitely take a close look at the feathering shaft. I bent mine after I had landed and the rotor had spun down to almost zero. It then tipped over on the skids and the blade hit the ground (again when it was on the ground and barely spinning). Had a similar issue plus a new vibration after that, but no visible damage whatsoever. Replacing the shaft fixed it.

If it does not look like it is bent (I couldn't perceive the bend in mine originally), try rolling it on a piece of glass (small mirror or a picture frame with a glass front... you don't even have to take the picture out). Roll it back and forth near eye level to see if either the middle lifts up or either of the ends. If it does, replace it.

Good luck... let us know how it turns out