After New Motor installation Thump Sound


I’ve just installed a new pool motor, new seal shaft, new defuser, impeller, new seal plate and all new gaskets. The pump is Whiperflo. I’ve done that before, but this time (after the pool was off for about 10 days) I hear some strange noises, like a thump once in a while in my pool pump, at the same time it vibrates a little. First I thought it must me air in the pipes, but after running the pump for about 4 hours it’s sill there. There is a enough water in the pool, no skimmer obstructed and the impeller is clean. No air leaks as well. The pump is runing fine, almost no air in see through plate, however it makes that strange thumping noise once in while. The motor is 2HP as the old one. Before I had the original Pantair seal plate, this time it’s made by CMT. Seem to fit okay, no leaks. What could be wrong?

Thank you for all your help.