1998 Chevy Silverado Idles up and down, whines around fuel

I just bought the vehicle and I had em put it ib the shop to fix whatever was wrong with it. The idle goes high and low, sounding like itand it wants to die out, but doesn't. There is a whinning noise around the fuel tank. I figured it was the fuel pump but the shop said it was fine that it just needed gas! So I put gas in it to appease them and it still does it. Obviously there is something wrong with it but apparently they don't want to pay to fix it. Am I right about the pump or is it something else?
Thanks for your help.