I have a Citroen Xsarra Picasso (10 years old) and have been driving it for over a year. Yesterday, I drove to town and parked it up for a couple of hours in an open-air carpark. I then drove about a mile down the road to another carpark. When stopping the car in a parking space, I heard a crackling and popping/bubbling sound coming from underneath the bonnet (driver's side). It was really loud and I've never heard it before (actually thought the car was going to go bang!). I left the car for around 20 minutes, started it back up and the noise began again. As I drove it out of the carpark (drove for around 100 m), the noise stopped and it was fine all of the way home (even stopped off at Argos to pick something up on the way back and no more sound). I've checked the fluid levels and they are all fine. I've been driving cars for over 20 years and this is the first time that I've heard this in a car. Does anybody know what could have caused this sound and is it serious? Thank you.