Ford Repair: tapping noise in 97 explorer engine, pep boys, chain tensioner

Question
I bought a 97 Explorer XLT 4wd at auction for $1325.  It has leather interior and power everything.  The interior and power options all work.  The engine has 140k on it and I knew it had a tapping noise before winning the auction.  I figured even if I had to drop a new engine in it that would be okay.  Well...looks like I might have to.  I am hearing a tapping that is louder in lower gears and hard to hear at higher speed.  I thought it was a valve tap (which I was told by Pep Boys).  Then I had another mechanic say the noise was coming from the bottom of the engine and it was a (chain?) of some sort that Ford Explorer's had a problem with.  Anyway, I plan to drive this until it dies and then make a decision.  What is your advice on finding out exactly what is wrong with it and should I invest in a new engine or rebuilt? Thanks.

Answer
Explorer 4.0L SOHC engines are well-known to make a ratcheting/rattle/grinding type noise, this is caused by a bad camshaft chain tensioner. If a tapping or knocking type noise is heard, it could be a valve, or worse.  Without hearing the noise, I would bet it is the camshaft chain tensioner.  There are two camshaft tensioners on this engine, one in front and one in back.  If both are bad, it is a fairly expensive repair, between $1000-$1500, because the engine may need to be removed for repairs.  You could swap the engine with a used one because of the mileage and issues with yours, but try to find one with the updated tensioners. You don't want to be back where you started from.  You could continue driving the vehicle, I have had vehicles drive in with tensioners so bad, you'd think the engine was falling apart.

I hope this helps,
Dave