Need help here.

I thought it was something simple but I couldn't figure this out last night. I just got done with a 3AT - > 5 Speed D/R conversion. Everything works fine. However, when I accelerate and then unload the tranny (push in the clutch) the tranny unloads and bumps the dog house. I can feel it hit and hear the load whump. This only happens when I pop the clutch fast, when I shift smooth its fine. I checked the tranny mount and its nice and snug, but I can push the tranny up and down a bit. All three engine bolts are tight too? The rear tranny shifter support is tight also. Anything else that could cause this?

make sure the tranny mounts are cool. no cracks ect. also the little strut on top of the trans to the body?

to what rallyruss says, that strut on the top my be out of adjustment, thats what mine did after an engine reinstall, I was able to see the engine move around with the hood open and having the ebrake on let the clutch out a little in 1st gear. That stut stops the movement if properly adjusted.