OK, this might be vague, but I get a rattling/rapping noise (not valve related) from my 1978 300D's motor.

sounds like "tang tang tang tang"

Here are the symptoms:

especially prominent when not hot, gets a lot better once hot, but not totally gone

only happens when you are starting off, once you are going 40 mph or more, you can even flog it and it's smooth/silent/non existant.

could this be a cracked or damaged prechamber?

could a faulty, incorrect, or leaky injector cause this?

any thoughts appreciated!