The alarm is the monitor for low oil, low gear lube and over temp. The reason it comes on with the ignition on but the engine not running is because there is no oil pressure. You could temporarily remove the wire from the oil pressure switch to silence the alarm and reconnect when you have completed the work you are doing but i don't recommend leaving it that way.
1997 2400 SCR
5.7 Vortec / Bravo 2