New clutch help!!

I put a new clutch in my 400ex put it back together and took it for a ride. I was in 3rd pulled the clutch in and downshifted at about 20mph into second. Here is the problem when i down shift through all the gears it sounds like something hitting the case:icon_cussing: The shifting goes fine up and down throught the gears except for the bang! any ideas ??????