My 95 Villager, 3.0 Nissan engine, bought used at 62000 miles, developed a noticeable heavy rattle at about 95,000 to 100,000 miles. I took it in at 115,000 to replace timing belt (don't neglect this). Dealer mechanic called the racket "piston slap," & said it is result of fewer lube ports in engine than were put into subsequent models. Sounds crappy, but hasn't affected performance; mileage now at 160,000. By the way, when you buy used, always inform the factory how to reach you. There had been a warranty extension on this engine to 100,000 miles. or 7 years because of this problem, but I never heard of it. I could have gotten a free engine!
We have a 95 with 128000 miles. Occasionally the check engine light would come on when cruising down the highway and it started to idle rough. Our local garage guy put it on the check engine machine and concluded the crankshaft bearing or the main bearing, or maybe a piston bearing or sleeve was warped. A week later the light went out, it is running like new again, and we didn't do anything to it! We think something was just plugging something somewhere and it cleared out. The check engine light does not come on at all anymore!
Regarding that engine knocking problem, my 93 Villager had similar issue. After some investigation I figured the noise comes from exhaust manifold. This is a very common Villager/Quest 93-98 problem. One or more exhaust manifold studs (usually from the right side of the engine) breaks, causing to knocking noise and leaking smoke. The noise will increase as the engine ages and the smoke will be annoying when the heat is on.
My crankshaft broke, and the engine simply would not start. I purchased my 93 Villager from my cousin, drove it 1000 miles to get it home, put another 100 miles on it, woke up one morning and it wouldn't start. The starter worked, so I thought the timing belt broke. I popped the distributor cap off and the rotor wasn't turning while cranking the starter. I went to pull off the crankshaft pulley and it just fell off into my hands... completely severed from the crankshaft. I never heard any abnormal sounds prior to the van not starting.