I bought a 2003 Xterra with 175k miles and a month later it began making horrendous noises. Sounds like someone left some silverware lying around inside it Mechanic says it's coming from the crankcase and is probably a spun bearing. Belts were removed temporarily to make sure it wasn't an accessory. A stethoscope was used to pinpoint the source to the crankcase. Can anyone think of what else it might be before I spend $3000 on a rebuilt long block? Oil pump? Damper pully? Transmission? Or do I trust the mechanic and replace it with a rebuilt long block?

Are you a DIY person? If so, pull the oil pan and check it yourself.

If not a DIY person, then trust the mechanic or get a second opinion from another shop.

Either way you decide to go, it's time, talent, or treasure you are going to spend. Good Luck and please post up what you decide to do.

So I was about to drop $3500 on a rebuilt (plus a hoist, gasket set, fluids, etc) and swap it myself. I got the garage cleaned out and organized my tools, took the hood off, started removing the radiator & belts. I stopped and literally stared at the engine, thinking about all the work and money and thought "Maybe I should get a second opinion". So I put it back together and headed off to a local shop. I noticed the sound was greatly reduced, but still present. Hmmmmmm. I don't think a rod or crank bearing is going to get better temporarily. Could this be a simple stuck lifter or something like that? So I turned around and headed to NAPA instead and bought some of that EPR gunk-dissolving oil additive. Ran it for 10 minutes, then changed the oil. PURS LIKE A KITTEN!
Best. Christmas. Ever.