whataretheodds

i bought a prothane trans mount and i replaced it today as i drove the car and when i get off the gas and as speed goes down i hear and clunking noise, it sounds like a thumping noise, all the bolts are tighten, trans member is even on the chassis, i dont understand why its clunking when i let off the gas?? it wasn;t like that today as i drove the car with the stock transmount, only thing i did was replace the mount and thats all and i even put a piece of rubber between the mount and the trans member but still i hear that thumping noise, what can this be? ive looked it over many times there's nothing binding up, bolts are all tighted up, does the trans mount need to be broken in?