Help with knocking in rear wheel

GJC

Hi I have recently purchased (2 months ago) a new wheel set flow ex on telsa hubs and new cassette, i have noticed when free wheeling the the cassette seem to catch and stop slightly in a frequency of every revolution, there also seems to be knocking some times when pedaling but this is less frequent.
Any help and adice would be greatfully recieved

Many Thanks GJC