Help! Engine fan draining battery after cars switched off -

[boombastictiger]

On my 1.8t Passat, after it is switched off one of the fans in the engine bay comes on and off intermittently. This caused the battery to die after a few hours so i called breakdown out and he didn't have a clue what it could be! The engine light was on so he reckoned something to do with this. The fan is not for the radiator and has a loud hiss which comes from near the turbo but seems to be behind some covering so cannot see it.

Does anyone have any ideas what it could be?

Thanks

Sk