Knocking sound coming from under engine or trans when in idle ?

I had a similar problem with my 94 Rodeo 3.2L. Many mechanics told the owner before me that the engine would probably have to be replaced. After I bought the Rodeo, I had the hood up listening to the tapping noise which sounded to me like it was coming from the timing belt shroud. I pulled off the shroud and started it back up and noticed that the belt was not tight and the tapping noise was the belt hitting the plastic shroud. Turns out that the hydraulic tensioner for the timing belt was bad. I replaced the tensioner and have not had any other tapping or knocking noises since. Hope this helps u.