owain glad to read you got your problems sorted. I'm adding some info for future readers who may find this thread via a search engine.

After doing work to my OM602 (91 300D) that included replacing my engine mounts and having the injectors rebuilt (by a local Bosch certified service center that does tractor trailers) the nailing noise had only decreased slightly.

I pulled the service manual and followed it to set the idle speed. It's now idling at ~800 RPM vs ~550 RPM and my nailing noises are 90% gone. An unexpected benefit is an increase in power to boot.

__________________
This car had a history, now it has a future.

Learn more about my Benz here.