Unknown problem....

Whenever I step on the brakes, there is a slight “thunk” sound from somewhere in the engine bay. The car drives just fine, but it sounds like something is shifting around on acceleration and deceleration… Anybody have any ideas at all as to where I should really start looking for loose parts? I have already pushed and pulled the motor around as much as possible, and I don’t think that I have a loose mount. Any help would be GREATLY appreciated!

Problem found… Loose lug nut on the drivers side!