So I've brought a merc vito van with a few faults, most of which I've fixed. One thing that is confusing me is a slight pinging/crackling sound only present at wide open throttle from 1500rpm to 2000rpm. It sounds exactly like an old petrol car with ign timing advanced too far being floored at fairly low revs. Just a slight pinging sound, not violent knocking or anything. Would not seem out of place on an old diesel but serms strange on a modern(ish) 07 common rail diesel. Noise goes away as soon as the turbo kicks in. Engine seems to otherwise run perfectly with no smoke. Noise very much seems to be engine related but can't rule out clutch or other ancilleries. Only thing i can think of is a poor spray pattern on injectors which doesn't matter most of the time except low rev high load. Any other ideas?

Put some 2 stroke oil in with your next gallon of diesel..
If it sounds better - buy your diesel fuel at a better garage

I'd be getting the plastic cover off and seeing if you've got any Black Death build up from a chuffing injector. You might not be able to hear it once gets well coated in coal but the management will be trying to compensate for the pot that's leaking. Leak off test would be next if it's not got a load of coal under the cover