Wierd Knocking sound from Engine on start up for about 5 sec

sounds like a faulty glow plug ,the knocking noise would be un burnt fuel at a guess , could be a dodgy injector on cold start dumping too much fuel in, if the glow plug no wormy proper then un burnt maybe

the engine heater light is telling you that you have faulty heater plugs....there should be some smoke as well which clears when your engine is running properly on all the cylinders..continued use in this condition will allow unburnt diesel to wash the walls of the cylinders that have failed heater plugs..and dilute the engine oil.... the results of which could get very expensive to rectify!!

HI Heater plug threads tend to carbon up inside the head and are easiest removed when the engine is hot.... socket and ratchet bar is the best approach unless you have the special MB tool.and inline leverage.they can pick up the thread in the head if you use to much muscle...feel one tighten.. screw it back in a bit and use quarter turns backwards and forwards until it comes freely.....

They were not much from MB £6 or thereabouts for Bosch/ beru. But any parts supplier should be able to get them. A suitable mechanic is probably more important to find. I can't help in London

MB offer a special tool but a comprehensive socket set is all you need...... along with a gentle touch.......lots of heater plugs advertised on E'bay but **Bosch/beru is the MB recommended brand type 0100 221 162 / 0250 201 026
** quoted from the MB service publication

good numbers for the 300 series engines if you go to GSF just quote the number they do not have any awareness of G'wagens

Hi you must remove the airfilter box to access the heater plugs which are fitted into the cylinder head in a line from front to back underneath the induction manifold and linked with electric cables...remove the nut that holds the cable to the plug and with a 12mm socket and ratchet bar remove the glowplug by turning anti clockwise.best to do one at a time to avoid any confusion....watch for cross-threading when fitting the new glowplug.a light film of oil on the threads helps you feel if you have not aligned the plug properly...damage to the threads can result in having to remove the cylinder head for thread repair....