The car is a 1977 300D/ W123.

The knock is not high pitched really, but kind of a clank-clank. It sure doesn't seem like a hose or belt slap sound, but I guess I could check.

I'll also try pulling the injectors or tubing to each cyclinder and seeing if I can narrow it down or eliminate the sound at idle at all... The guy I bought it from said he thought it was the number two cyclinder, so the cylinders are counted from facing the fan/radiator and back towards the firewall, right?

Thanks again all.