2004 mondeo, knocking sound from back, don't know the reason. ?

Knocking sound from back of my mondeo 2004. In my last service the mechanic told it's not to do with shockers or it's not impacting performance but it's very irritating noise.

My questions is to find out what's the exact reason is, ford is charging investigation cost of £100 +vat and then they will get me a quote to fix the problem.

let me know if there is any other cheaper way.