strange beeping noise outside of car

[99blueb5]

every once in a while i can hear a "beep...beep...beep...beep" outside of my car ussually it starts pretty slow then it gets faster and faster and about after 25 seconds it stops. you can hear it if the car has been running for about 10min at idle from a complete cool down and usually after the car has been at idle from driving over like 20min, what is weird is that i can trace the noise back to the divers side right before the rear wheels near the pinchweld someplace and when im that close it sounds like a valve that is letting air out when the "beep" occurs, so im assuming its the tank purge valve? any ideas, im pretty sure its not supose to beep like this, the beep isnt to loud but you can hear it over a conversation and i get the occasional "uhm does your car have a bomb on it" from some people

[Seerlah]

The purge valve is located in the engine bay. I suggest to locate directly where the beep is coming from.