I did check the exhaust system for leaks recently and all was good. That was one of the things that came to mind.

This had come to mind thinking of worse scenarios. I run Delo 400 which is a good diesel oil. I could try good ol' STP if that's still around. I remember using it years ago to quiet the lifters in high mileage gas engines and it worked good.

Run one of the good injector cleaners through it when I don't do this every few fill ups my SD gets louder..............
William Rogers.........

It was a shot in the dark sometimes the injector pump can make a noise and as it is tied to the rpm of the engine it will be in sync with acceleration and match your engine. If I remember correctly the vacuum pump is also right behind the injector pump, so either could cause the noise as they both are connected directly to the engine and will vary with engine rpm. The hard part is isolating the sound, I think you are on the right track there to fix this you are going to have to pinpoint the sound exactly, not easy on an MB diesel.

I ran two cans of LM Diesel Purge last Saturday with no improvement on the noise problem. I thought it was running better immediately after but I guess that was when it still had the diesel purge in the filter. It still runs the same which is good.

If it is in the IP or the vacuum pump, then I won't worry about it. These items will not be as serious as an internal problem if they should quit. An IP or v-pump won't cost as much as an engine to replace but an IP will still run a few $100 to replace.
Right now there is too much snow and ice to drive the 300D, so I'm back to driving my tried and true Tahoe and it'll go anywhere. The Benz can't get enough traction in the slush to keep the rear wheels from spinning.:(

I was in St. Augustine two weeks ago. I have had this annoying ticking or knocking noise for a long time. Anyway while I was there I pulled into a Mercedes garage a small independent shop. I asked the guy what it was. He said it was the vacuum pump. He said it was worn out and was causing this annoying ticking. He said he had changed many of them because of this. I will soon be changing mine.

My 2 cents

Dieselman

Interesting. Its worth looking into. I thought that since all my vacuum operated accessories worked fine, the v-pump was fine. When I was cracking my injector lines to see if the noise went away, it did or at least I thought so. Is it possible that when the car was running on one less cylinder at a time it was cancelling the noise in the pump? I'll have to check and see when or if my v-pump was changed. I wonder, how long is the pump supposed to last and is it worth rebuilding?

Consider a loose pre-combustion chamber might make such a noise.
Loosening the injector line could also identify this.
A friend discovered this in his 4 cyl a few years ago.

Is there a way to check the tightness of a pre-chamber? My guess this could affect the compression or cause some damage in the long run.

I thought it was a head-off job , Sorry it was a long time ago.

Maybe someone with a manual on CD can look it up for you.

Thanks, I have the CD FSM. I thought that there might be an easy way to check it.