Help with strange knock???

last night I had my neighbor come over and help me rock the various sides of the car and also lifted one side at a time to check a arms, and all joints for slop/ play. Everything seemed to be fine. I found a large parking lot near the airport and made a few runs while turning the motor off and coming to a stop. I did not feel the knocks while braking! So I am going to assume , at this point, that either something strange is going on with my power steering or i do indeed have the motor/ headers knocking something. With that said I have been all over the vehicle looking for evidence of the headers hitting something and I cant see it. I'm thinking my next step will be to install a torque strap to keep that motor in place and go from there. The one thing that baffles me is....Why would the motor jump when I apply the brakes????? remember its only at low speed non stressed braking