ack! im really not sure if the pump would be making a noise like this. hopefully its not a dreadful knocking (rod bearings), or you gf probably is right, that the car is about to die, but it probably would've already done so if this were the case. i know you said it starts when the light comes on... does it continue until you turn off the car? does it come on while driving/moving or at idle?

your best bet is to check the pressure first to see if it actually is losing pressure. if the pressure is NOT low, then the switch is probably just going bad (not sure about the horrible noise in this case).

if it IS losing pressure, then i would say to first check the filter so see if it's clogged with "gunk", obviously replace if it is. i think the oil pump also has some kind of "screen" to catch debris, you may want to see if it is clogged with gunk too. while youre at it open the pump cover and inspect for any damage. obviously if the pump IS damaged you'll want to replace it.
if the pump looks fine then you'll probably want to start checking the main bearings for wear and replace them if needed. i've read to also measure the rod and main bearing clearances, but honestly i dont know much about this.

it is odd that it runs fine 99% of the time though.

anyone else have any suggestions???