buzzer alarm

i have had a 1991 jetta 2 years now and when ever i go on long trips (5+ hours) this buzzer alram goes off and i dont know what it is indicating. it usually happends at the top of a hill or on hot days. and recently i will be driving for no more then 20 min and the same problem occurs. when his occurs i try 3 things to stop the noise( this noise is not a mechaical one, it seems to be some kind of buzzer) either i take my foot of the gas peddle and reapply the gas or i down gear (i have a standard) or if none of those work i turn the car off and when i turn the car back on again the noise stops. the engine temp gauge is always normal when his happends and is not over heating. no indicator lights on my dash light up when his occurs and the car doesnt drive any differnt then if the sound was not on. so if you could help me to find out what this buzzer alarm is trying to tell me that would be great
thanks

Soundsl like the low oil pressure buzzer. When you turn the key to on w/o starting the engine does the oil pressure light come on?