Renault Grand Scenic - engine knocking noise

Last week the engine of our 2005 1.5dCi Grand Scenic started to make an very loud rhythmic knocking/tapping noise. The car appears to be running fine with full power but is making a racket. A local guy that dabbles in cars had a listen and thought it sounded like a serious engine problem - was talking about the "big-end" etc. Its now with the garage and they are saying that it needs 2 fuel injectors - likely to cost €1000. They cant say that this is the only problem though - could be the start of the spending! Does this sound plausible - anybody know if a car with a couple of dodgy fuel injectors could make such a noise?
thanks

injectors cost alot to replace as the engine has to be taken apart from what i know. also if the injectors break and they can get sucked into the engine proper then it can ruin the engine altogether

I'm not a Renault expert, but the problem (which apparently is not unusual in these engines) sounds like it could be linked to a pressure regulator valve on the common rail direct injection (CRDI) diesel engines used across the Renault range. These use high-pressure injection (1.0k to 1.8k BAR).

If a mechanic touches the injector rail and it pulses / vibrates in sync with the knock, then I would be inclined to suspect the pressure regulator valve. I have no idea if these specific valves can cleaned / repaired or what they might cost to replace.

If you can, try and get a decent independent diesel expert to check the car rather than a main dealer.

Thanks for the information folks.
Just to update you on the situation. The garage have sampled the diesel that was in the tank and it was filthy. Looks like a muddy sludge in the bottom and seems to contain water as well. We always buy our Diesel from a heating oil supplier in the town who supplies half the town with petrol and diesel as well. I rang them today but of course they are saying that they have had no reports of problems from any other customers.
The garage has now changed 2 of the 4 fuel injectors at a cost of €1000. Unfortunately, it hasnt fixed the problem completely - noise is less but still there. I have a feeling that this is going to cost me a lot of money!
Speaking to some people, they feel my insurance should cover it but I have my doubts.

A muddy sludge or water contamination in a diesel (or other fuel) tank is not that unusual - it's the reason cars are fitted with fuel filters and on some cars there are sensors to indicate that accumulated water needs to be decanted from the fuel filter housing. What state was the fuel filter in?

I cannot see how motor insurance can cover this problem.

Renault Grand Scenic very loud rhythmic knocking/tapping noise

[FONT=&quot]After 5 minutes driving my 2010 Grand Scenic 1.5dCi [/FONT][FONT=&quot]started to make a very loud rhythmic knocking / noise while driving in second gear. When driving on third gear the noise seemed to disappear. The noise sound comes from the engine and i was wondering if any of you would have seen this problem before. Any help would be much appreciated before sending to the main dealer tomorrow. [/FONT]