vibration or knocking on tick over
 

Hi All

My wife has a 2003 mercedes a170 cdi auto, it has a knocking noise on the right hand side on tick over, it goes away under normal driving and if you take the revs up slightly its fine, the exhaust was shot so i changed it for a new one the bracket on the back box had rotted away so exhaust was moving about on tick over i assumed it was that but noooooo :confused:.
checked all underbelly covers tightened and replaced missing bolts it wasnt vibration off that either :(, i cant see anything loose in the engine compartment, i am at a loss to where its coming from, any ideas or anyone had a similar knock....cheers in advance.