Kenneth:

You're probably just in need of an oil change.  The light/buzzer, I
believe, indicates either too high of oil pressure or too low. 
Although, it can be an indication of a more serious problem, given that
you state that the van's been parked for 3 months, the oil probably just
got a little thick (or thin) and needs changing.  The light/buzzer has
come on in my van maybe 3 times since I've owned it and all 3 times it
had been a little over 7500 miles since an oil change.  Changed the oil
and no more peeps.  If you change the oil and the light/buzzer continue
to go off, more investigation is in order.

Hope this helps,
Tonya
87 Vanagon GL Wolfs
74 S. Beetle