311 cdi. diesel knock / rattle

Can any one help my 02 311 cdi (316k) has developed a knock or rattle every now and again. Most of the time it starts and runs fine . it seems to happen when you are going along idling the engine and you increase the revs a little. It will hold back (sometimes feels as if missing) and then knock (just once) and a puff of black smoke then it's fine . It will do it at the start of a slight incline with the revs around 2500 . If I gear down and race the engine its fine. it only happens at the first touch of the throttle.
I did have the limp mode problem before this. was going to change transducer vac and wiring. Does any one know if the transducer vac can get worse and cause this knock ?

Going to have it plugged in on monday But hope some ones advice might save me the cost
Thanks steve