When idling and pulling away my backbox knocks/ rattles terribly against 'something'. Worse when cold.
But finding impossible to diagnose the 'something'.
With car on the ground, I have reached as far as possible and can't feel any contact. One pump on jack/ up on bridge and it stops/ doesn't knock.
Given that one pump on jack(10mm raise?) stops it, tried over-inflating tyre for testing - no joy.
Definitely isn't against shocks.
( car lowered 30-35mm, backbox GpA twin pipe).