98 trooper 3.5 Knock while reving comes and goes

Just bought a Isuzu Trooper 98 with 158000 looked clean idle good had a very slight tap didnt worry about it because it needed an oil change. On the way home enginge started knocking. I got home changed the oil. It idles perfect very quiet then when you rev it it knocks at lower rpm then sounds normal. It sounds like one cylinder. The check engine light came on. i pulled the battery checked all the spark plugs cleaned and re gapped ther ngk they do need replaced. staarted the engine which started faster than normal and it purs like a kitten tell you rev the engine and one cylinder knocks. did a test drive. it only knocks under light throttle.
Any help would be appreciated i bought this for my wife who is do any moment with our second baby cause im having issues smogging my diesel well i got screwed.:mad:

Could be a weak timing belt hydraulic tensioner.

Is there any record that the belt, pulleys, tensioner and water pump have been replaced (routine items @ 100K miles)?

You'll find lots of Trooper info and support on http://forum.planetisuzoo.com/