Sorry for the long message but I want to give as much info as possible, in hopes someone might be able to give some help

I'm a relatively new owner of a 1990 350SDL with 210k miles. I'm also new to owning a diesel. The car is in excellent overall condition and I have service records showing that it was well taken care of since new. It starts and shuts off immediately as it should, uses no oil, does not smoke, has no blowby, seems to have good power and all vaccum functions seem to work properly.

My problem is that there is a quite loud engine noise at idle, and off-idle, from the top of the engine that I think, from reading the posts here, is injector nailing. It does not seem to be coming from any particular injector.

The odd thing to me is that when starting a totally cold engine, the engine is very quiet for the first short period of time before the loud clattering begins. The colder the weather, the longer the engine is quiet after starting. Last night the temperature here got down into the 40s F. When I started the car this morning the engine was very quiet idling for the first 45-50 seconds then the noise started and stayed loud the whole 15 mile trip I made today.

I tried the diesel purge trick of running the engine on Liqui-Moly and the engine quiets down but the noise returns once the Moly is gone. The noise also seems to go away once the engine reaches about 1100rpm or so when I rev the engine while sitting out of gear. There is no detectable engine nailing (if thats what it is) during normal operation unless rpms fall down to around 1100rpm and below,.

What could be causing this? The noise is quite loud and I'm worried about driving the vehicle until I fix the issue. Any advice would be greatly appreciated.

Thank very much