[Syndrome Zed]

Hi all, long-time lurker, first-time poster and all that jazz...

And also a recently-acquired Metro owner - a 91, standard G10 engine and manual transmission. The little bugger's been running very well up until recently, getting me about 42-44 mpg and saving me a fortune as I have a 110 mile commute each way, 5 days a week. Yeah, I know: But my sanity in taking on this commute aside, I have a problem now and need some advice.

Last Friday I was driving home and stopped once on the way down. I'd had the radio on so I don't know when it actually started (other than on that trip), but when I started the car back up to finish my trip, I noticed it had developed a pretty loud knock. I'd checked the spark plugs a little while ago and cylinder 3 had some oil fouling - I knew the car burned oil already, but not very badly. Well, the knock's in cyl 3 too. It sounds similar to baseball_playa's knock in the video he posted, I'd say. So my first assumption is rod and/or bearings. I can't get a good lock on where the sound is coming from, other than the one cylinder. And the weird thing is that the sound drops off a lot, or even disappears, when I'm coasting at speed (65-70 on the highway). It does seem to be quieter when the engine is hot (ie idling after a long chunk of the commute), but it doesn't go away completely. I haven't noticed any loss of power, there's no misfiring, and since it's my only source of transportation, I've had to drive it in spite of the knock.

So here's what I'm wondering:
1. Is there any way to tell whether it's a lifter, the piston, the rod, or the bearings, just by when the sound is louder/quieter/gone? I tried the "mechanic's stethoscope" bit, but I couldn't get it localized any better than that cylinder.
2. I went ahead and picked up a set of rod bearings just in case. If all I'm doing is replacing the bearings, how difficult a job can I expect this to be? I've been following the exploits of DOCTORBILL and it sounded like removing the bearing caps and replacing the bearings wasn't too bad, once he actually got them shipped. It's just a "hope it's either lifters or bearings and nothing else because I don't have the time/money to swap engines" kind of desperation move on my part
3. If it was something as bad as a bent rod or cracked piston, how fast would the engine become useless? Under duress from my boss, I've now put another 400 highway miles on the poor car. The knock's exactly the same as it has been, no worse and no quieter (except when coasting or maintaining speed). If it was one of the "terminal" scenarios, shouldn't the thing have gone by now?

If I had the time and cash, I'd be following in DOCTORBILL's footsteps right now, but as it is, I just want to keep the lil' bugger running as best I can. Am I just tilting at windmills at this point or is there hope for the wee bairn after all?