Its the buzzer from the switch back by the oil pump. Pull the cover off, pull the rubber plug out of the top with a pick or small screw dirver and screw the allen head in (clockwise) half turn at a time until the buzzer quits going off. Can't remember what size allen wrench but I think around 7/32. May want to take a size either direction under there with you.
'02 Silverado Z-71, STS Turbo @ 10psi, TRT Custom FMIC, Comp 222/218/115 cam, Patriot Gold Dual Springs, Hardened Push Rods, 60# injectors, 160 Thermo, FAL E-Fans, Nelson Harness, Yank 2600, Built Rock On VacMod 60E Transmission,Walbro 255 fuel pump, ASP Underdrive pulleys, Gibson Shorty Headers all polished up with a Nelson Street Tune