Originally posted by Hocky
Seems to be coming from the rear bottom(fuel pump/filter area) or the vacuum pump in the boot.smells.
Azhari, unless it was changed for your later model year car, the vacuum pump for the door locks is under the rear seat bench. It's a fairly likely culprit for A) doing things of its own accord and B) Making sounds like a fog horn, so it might be worth having a look there. I would try running for a day or two with the pump's fuse out, or with the power connector removed from the pump.
Remember to lock your doors manually! And you'll have no locking fuel flap either.