I'm looking to buy a 1994 Nissan pickup but it's

It sounds like you popped a bearing. You will have to remove the engine and strip it down to see the problem. You might even have bent or broken a connecting rod which is blocking the movement of the engine.

looks like that either you do have an air mass not correct working,or the throttle body is too dirty and you have to clean it.Also check for any cracked, snapped vacuum hose.

Sounds like a fuel pressure issue. Have you changed your fuel filter lately ?

I would suspect it is a wheel bearing issue, if it sounds similar to a baseball card in the spokes of a bicycle, stays somewhat same pitch just louder with speed that is where I would look. Does noise go away or change when making turns?

fuel pump or fuel pressure regulator

feul pump or alternator give the alternator a hit with a spanner to losen the brushes

Visit the on-line store, know as reliable-store, they have the repair manuals. I'm sure you will get the solution of your problem.
Nissan Maxima Factory Service Repair Manual 1994 2009

follow the given link,it may be useful for you 1997 Nissan Pickup Service Repair Manual

Replace "idle speed motor" (also known as idle air contrl valve).

here is how you can tell it is the fuel pump. turn your truck off. than have someone turn the ksy into the on position while they do that stand back at the end of the truck where the gas tank is located. when they key is switched to the one position thhe fuel pump should kick on and make a humming noise. if it is the same noise you hear when your ideling than that is you culprit.... my suburban does the same thing.