beeping sound under the hood after engine stopped
 

I noticed that my 02 ML320 (50K miles) has beeping sound under the hood after engine stopped. The beeping will eventually go away after a couple of hours. The beep sound is not loud. You can hear it only when you park the car in a quiet place. The sound is intermittent. I could not pinpoint the location of the noise. Does anyone know if this is normal? Thanks.