unusual noise

Hi all. I have an 04 Td4 which seems to have an odd noise on the right side of the engine. It comes on about 30 seconds after start up and at first I thought it was the fan kicking in. When I looked underneath the fans weren't on!
Its like an electrical humming noise which stays on after the ignition has been switched off for a good 2-3 minutes. The car starts and drives fine but its only recently this noise has become apparent.

Mine does this also. Sounds like it's coming from the cavity behind the passenger side headlight. It certainly sounds like a fan, although not figured out what it is yet. I'm sure someone on here will know!!

FBH - fuel burning heater

we get this every year once the weather get cold .
It was 5 years till I found out I had a FBH fitted to my TD4 .
There will be an exhaust pipe sticking out of the bumper on the passenger side , FBH works when temp is below 5 degs .