Car is a 1990 OM603 300D N/A Euro version

I know there is a ton of threads about nailing after an injector service, I havent found one similar to what I have going on.

Chasing a slightly rough idle, I booked my car for service at my local Bosch place.

They did injectors, balancing but not replacing nozzles as they were reported to spray fine.

Slight improvement in idle shake but now I notice a nailing pattern.

From normal idle, it will start nailing softly, get progressively louder, then go quiet. It will stay quiet for a few seconds, then start ramping up again, then go quiet again.

It's not that loud, my old 87 300D from way back clacked way louder, this is something you may even miss if you werent listening for it.

Would be interested in hearing thoughts on this.

An air leak in the fuel lines going to the Lift Pump can cause nailing and of course an uneven idle.

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84 300D, 82 Volvo 244Gl Diesel

I will check for this, thanks.

I'm not sure if this would be the main cause though, only the injectors were pulled and calibrated? They did not touch the pump.

EDIT - Prior to service there was no hint of nailing.