I have searched on the car and here to find the knock after my suspension was lowered. Its not the exhaust etc. One thought was my petrol gauge has been bouncing lately when low. Could it be the float has come loose and is banging on the inside of the tank?

Fill the tank.. if it still knocks... you have your answer

If the float wire has come away from the clips it could be rattling around all over the inside of the tank?
It's just that I'm going crazy with the knocking.
The rear brake compensator spring and rod are rattling at the top of the rod under the car. It could be that? Is it broken? The rod and spring work and the rubber Bush seem OK but the rod is badly rusted. Any ideas?