Thud a few seconds after accelerating after letting off gas

My car recently has leaked a bit of transmission fluid. I replaced the fluid and put some conditioning fluid in it but it is still doing something funny. No matter how fast I am going thirty to seventy mph whenever I let all the way off the gas and then hit the gas again, about three seconds later there is a thud coming from underneath my car. It is not as bad the harder I press the gas but it happens pretty much every time.