'89 Hardbody D21 4x4 question...

[xtjp760]

I was stopped at a light. I heard a loud noise from under the hood. Sounded like something was stuck or dying mechanically. It went away but then I noticed my fuel gauge was fluctuating. Is this an alternator problem? I also have just changed the belts. All three. I didn't notice anything else electrically going on though in the cab.