Citroen Xsara picasso 2.0 Hdi - Ther is a knocking noise immeadiately when l start my car and again when the ignition is switched off, pls help?

There r 2 usual suspects for this problem.

1) check the tightness of the bolts holding the rear engine mounting (underneath in the middle), try tightening them hard. If u get even a little bit of a turn on the bolt then it was loose.

2) try tightening the bolts that hold the subframe in place, these r the big ones that go straight up from underneath with an 18mm head.

Usually u will find u have a slightly loose engine mount bolt but if u find it was a subframe bolt then remove it, clean the threads & apply loctite before u refit it. A few Picassos had this problem for a short time. There was even an info rapid (bulletin) about cataphorises of the bolts.