I have a 1990 nissan pickup and most times when I start it up it makes a rather discomforting grinding/rattling noise then tapers right off after a few seconds after starting it. Any ideas of what it might be/ how I could get rid of the noise?

Possibly starter gears not disengaging after start up. Possibly timing chain from lack of lubrication from the vehicle sitting.

It sounds like yuo have a Bad starter motor, very common. The Bushings in them dry up over time then they will make excessive noise when you start it...goes away a couple of seconds after engine fires. I would reccomend replacing the starter.