My '84 300SD has been developing some noises for the last month or two that have me a little bit uneasy. I have some injector nailing going on, which is loud when cold and almost disappears at operating temperature. But that's not what has me a little worried. There's an irregular knock that's in the soup of noises, and I am pretty sure it isn't part of the usual diesel clutter.

A week ago, I changed oil and filter. Today, I replaced the primary fuel filter with a Hengst filter and pre-filled it with diesel purge. That seemed to quiet down the injector nailing some, but that irregular knock is still there and sounds like a loose bearing knocking in its seat or something...

I bought a cheapo mechanics stethoscope, which is kinda/sorta like what doctors use except with a long metal rod at its end. I poked around the engine and listened. It's a pretty neat device for sure.

I put it up to my injectors and tops of prechambers, and all produced the rhythmic ticking, some being quieter and some having more clattering soundscapes. I wasn't really expecting to isolate a nailing injector or a faulty pre-chamber this way, but now I am curious about how the healthy injectors/prechambers are supposed to sound through a stethoscope. Anyone know?

Anyway, back to what I termed the "loose bearing knock." My main fears are the timing chain and/or the vacuum pump failing. Where would be the best spot to listen for abnormal timing chain noises? I put the stethoscope up to the front end of the valve cover and did not hear any unusual noises. I put it up to the power steering pump - sounds smooth. I put it up to the vacuum pump - well, there was a little knocking I could hear, but since it bolts right up to the front of the engine, it made me wonder that maybe these noises just carried over from the engine. I put the stethoscope up to the water pump and heard somewhat louder knocking from it, but again - not sure if those sounds migrate from the engine block, or if my water pump is going bad.

Can anyone help me interpret what I'm hearing via the stethoscope and maybe someone knows how things are *supposed* to sound through the stethoscope... Should I also listen to the bottom end of the engine and see if the knocking could be coming from down there? What are the "better" spots on the engine block where I could put my stethoscope for listening to various parts of the engine?

P.S. When I was listening to the engine, it was right after a cold start - first start of the day. I did not have the chance to listen at operating temperature because it got dark by the time I got back home. Since the diesel clatter seems to subside some at operating temperature, I wonder if I'd have a better chance locating that "knock" then?

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84 300SD - bought in April '06 with 237,000 mi