Engine Knocking

So, i pulled up at the lights today and the engine made a very loud knocking noise, but when i accelerated away it stopped making the noise, it only made the noise when idling, i called the r a c and they put it on flat bed and took it to the garage, any ideas please what the noise could be caused by?

Let me get my crystal ball out.............................
Engine type?
Approx location of knocking on the engine?
Is there oil in it?

'54 td4 hse?

Engine 1.9 td4, hse, yes there was oil in the engine, knocking sound coming from cam chain area

Could be the auxiliary drive pulley on the end of the crankshaft. This is also the torsional vibration damper for the engine and consists of two parts connected by a bonded rubber bush. If this fails the pulley oscillates and makes a rattling noise. I thought my timming chain was on the way out when mine went. Look for polished areas on the face of the pulley in the sectors on the outer rim. A normal polished area of approx 5mm is ok as this is the normal range of movement. anything larger (10mm) and I would suspect the damper.

It was drive pulley at the end of the crankcase. Thanks,all sorted now.