start with a compression check of the cylinders. If it is as loud as you say you should be able to isolate the cylinder making the noise rather easily. THen you will need a valve job as well. sounds like you have some slop in the vavles or it is possible the crank shaft.

2002 Honda Passport. 3 inch Indy4x lift with 912 OME's. 265 75 BFG m/t's with a 3 inch body lift, safari rack, tube sliders, tube rear bumper, on-board air