You could post in the mechanic's forum. It sounds to me that the paws are getting hung up. That causes the pedals to turn when the wheel is spun. What I cant explain is the vibration. Remedy is to lube up the freewheel hub. Do you over clean the cassette? This can lead a dry freehub and the paws get stuck. This happen to me and I would get chain suck up. In my case the hub was not serviceable so I needed a new hub. So I don't near the cassette with the orange cleaner any more.