@Kev K - I'm going to be brutally honest with you.
The timing chains are for sure the Achilles heel of the 4.0L SOHC engine. This is well documented. The problem that leads to the death of the engine is timing chain cassette failure. Timing chain rattle starts with hearing rattling at cold start. This can be due to the timing chain tensioners. The tensioners rely on oil pressure to tension the timing chains. As oil pressure isn't available until after the engine starts you get rattling until oil pressure fills the hydraulic tensions (one for the front timing chain and one for the rear timing chain). If the problem is the rear chain some have installed a manually adjustable chain tension. This tensioner does not rely on oil pressure to take the slack out of the rear chain and hence eliminates the noise at start-up. If the noise is in the front chain, you can try replacing the front hydraulic tensioner, but depending on how much slack is in the chain replacing the tensioner may not help. With age chains stretch and the longer the chain the more it stretches (the 4.0L SOHC has 2 very long chains) so even if your chain cassettes are still in tack you may have noise. Once the timing chain cassettes break your engine's days are numbered as eventually a chain will become so loose it will jump-time (or even break). It is not a matter of if this will happen, but when. Also, as someone previously stated, the broken pieces of timing chain cassettes will block the engine's oil pickup, which will cause a loss in oil pressure.
The SOHC engine with it's long timing chains and use of a jack-shaft to spin the right side's cam plus the use of hydraulic tensioners is a poor design. Some engines experience TC noise with as little as 75,000 miles (or less). Some report going 300k without issues, but these people seem to be in the minority.
As far as what it will cost to replace the timing chain cassettes once they've failed, it depends on which cassette has failed. This is because the engine must be removed from the vehicle to replace the right side cassette. A complete timing chain component replacement job is typically in the $2300 range. The left cassette can be replaced with the engine still in the vehicle, so that's a less labor intensive job. Same for the front jack-shaft chain tensioner.
So, what will it cost to eliminate your rattle depends on where it's coming from and what's causing it. The least expensive fix is to replace the rear hydrualic chain tensioner with the manually adjustable chain tensioner, which is a Polaris part and runs around $50-$60 plus installation and is fairly easy to do.
If I were inclined to put money into a 22 year old Ford 4.0L SOHC engine, which is rattling, I'd begin by removing the lower oil pan and see if you find a broken pieces of tan colored plastic. If you do your timing chain cassette(s) have failed and you're looking at a pretty big job to a huge job depending on which cassette has failed.
Let's assume that you spend the big bucks to have both timing chain cassettes replaced (plus incidentals) that you use quality replacement parts and that you can find a qualified mechanic to do the job for around $2300. Your next area of concern with the V6 engines is the somewhat weak automatic transmission, though the 97's 4-speed trans may be a bit stronger than the 98-forward 5-speed trans. A trans rebuild will run you another $2200 or so.
Frankly, I don't believe putting over $2000 into a 22 year old vehicle is worth it. I also believe that owning a 22 year old vehicle is a bad choice if you have to pay someone to make repairs to it.
IMO you got burned buying a SOHC V6 Explorer, but you're certainly not alone. Ford continued using this engine through 2010 w/out ever fixing the problems with it.
BTW - A remanufactured SOHC engine runs around $3900 + labor and used salvage yard engines can be a bad choice because all the SOHC engines ('97-2010) suffer from the same problems.
It's not clear if your saying you only paid $600 for this vehicle. If that's the case I'd just drive it into the ground and then look for another $600 vehicle. Who knows, you might get a year+ out of it.
