knock knock... what's that?

[cpatstone]

First an intro: just bought Gimp's car. Flew down to BWI and drove it back... long day, but well worth it.

Shortly before I bought it, the car developed a bit of a knock. This was fully disclosed. PO and his mechanic suspect rod knock. I'm not sure, or at least I'd like to eliminate other possibilities before pulling the motor.

Symptoms: essentially no noise at idle, and up to maybe 2500 rpm. 3000+ and it's there. somewhere around 3000, it sort-of hit or miss happens, but on a per-revolution basis, it's either there or not. So it's not like a ticking that gets progressively louder with higher RPMs.

CEL is on, but for an oxygen sensor which we think is unrelated. At the higher RPMs, it's not subtle... sounds really bad. I wasn't sure I'd make it from MD to MA, but I limped along and it did fine. Got great mileage, too. Not overheating. Don't know if it's lost any power, but the PO didn't seem to think so.

Motor has 184k, well maintained but not necessarily babied. PO changed the oil after this started happening, ran the oil thru a coffee filter, didn't see any metal particulate. I assume he used his pela oil sucker, so there might be a little junk in the oilpan that didn't get picked up with the oil. I guess the oil filter looked fine too.

Had a buddy over tonight and he did the dowel trick: one end on the motor, other by his ear. He thought the knock was more top-end. He thought there might be something glaringly obvious -- and the noise would make you agree -- but I just pulled the valve cover, and all looks fine. No metal particles at all. No visible scoring. springs seem fine. chain looks fine.

Fully disclaiming that I'm a complete noob to the M44, I'll throw out a couple of thoughts and I'd appreciate any comments.

* Timing chain guides are plastic, right? do they break (think M62)? if that happened and the chain was flopping around enough to really whack the engine case, wouldn't the chain skip teeth and become a really big problem?

* Apparently the valve guides can wear and/or break? If a valve guide was causing excess drag, would it cause the valve to not return as quickly as it should, thus not following the cam, and whacking the cam when the two caught up?

So? thoughts/comments/suggestions? Thanks in advance!