99 Explorer Sport Engine Noise

I have a 99 explorer sport OHV. It has 42,600 mile on it. I recently changed the oil and transmission fluid and just put in a new set of spark plug (Bosh Platinum 4+) today. I have noticed that the acceleration has declined some in since installing the new plugs. My main concern though is a knocking sound that come from the engine. It is very erratic and doesnt start until about 30 seconds after cutting the car on. You can only hear it though when you pop the hood. Does anybody know what this could be?

I flushed my radiator today and that fixed the ticking problem. I dont know how, I am assuming it had something to do with the water pump though??? I bought 2 bottled of preston radiator flush and flushed the system out with that, then I flushed it out with pure water a total of about 6 times. The water that came out the first time looked like something you would see in a mud puddle. After about the 4th time it started to get cleaner, by the 6th time it was clear. I installed DEX-COOL Extended life Coolant, Ford says not to use it but every mechanic I have talked to said that it would be fine as long as the system was flushed through really well. I dont know though why the coolant was so dirty, I bought the vehicle at 22,000 miles and I think it was flushed before that. Can it really get that dirt after another 24,000 miles???

I have no idea if this was/is your problem; judging by your apparent solution its probably not the same thing...but I had a similar knocking noise in my '98 OHV sport. It started 30-60 sec after cold start and wouldnt go away...it ended up being something drastically wrong w/ the timing chains, which in turn led to a failure of the crank bearings. End result: a $5,600 bill for a new engine and labor. So be wary of that knocking.