OM617 knock. Where to start?

[wstetson3]

OK, I bought a 300CD from a fellow forum member who was completely honest about the knock in the engine. Honestly, I bought the car to drive for fun while I am working on the 81 N/A and doing the manual conversion. It will also be a second donor for parts should I ever need anything. It has the black interior, the same as the 81 I am working on.

Well, the car arrives (he drove it here, about 4 hours) and there is a slight knock. The odometer shows just over 200k, the clear fuel filter is clean. It SOUNDS like it is coming from the back of the engine. Of course it COULD be coming from the crank but I'm hoping it's somewhere on the top end. It isn't loud but you can hear it. The PO isn't sure where it is and we agree that it could be a number of things. He was honest about the car right out of the gate. Oh, it doesn't like to idle until it's warm. Once it's warm is seems to run fairly smoothly. Not like my 81 or my yellow 82, but not too bad either.

I have an 82 I am using or parts already so this would be a second CD parts car, if I ever need it. Parts for the CD are rare enough that I wanted to hoard some parts if I ever need them. I'll put liability on it and drive it until then, assuming the engine isn't terminal. I COULD drop my other OM617 in it as well. I was going to put it on an engine stand in the garage.

Side note: The guy shows up with a 450SEL 6.9 Euro on a trailer. He was delivering that one too. His wife was driving my 300CD, he was pulling the 450. It needed to be repainted and some work to be a driver but I would love to have had that car! I've never seen a 6.9 in person. That thing was ridiculous. Mercedes went all out when they built that car. I do love the 116 cars.

Where would a guy start looking for the knock? Any suggestions would be appreciated. I know it could be an injector.