Agree... remove valve cover and see whats going on. Also pull the spark plugs and look in with a flash light and see if its rusty in there or what?

If its over 100k miles I'll take a guess at balance shaft seized up.

Want an easy swap/upgrade... starquest 2.6 is as easy as it can get.

88 Montero, StarQuest turbo, Kormex 5speed +tuff-pan w/phoenix f. stage2 clutch, 4in lift+33' tires, 4.875 lsd, short-throw shifter, Aisin hubs, SR big brakes, gen2 uca's, elantra alt, and so much more... 16g turbo w/hardpipes + 4.90 locking disk-brake axle soon (maybe lsd or aussie front??)