JIM R

Hi there, I have a 2006 535d tourer ( wagon) which has an interesting problem.When I switch the engine off and leave the car you can hear a hum coming from the engine which lasts for anything from 10 to 30 seconds.For a long time I thought it was normal and ignored it but recently took it in to have it checked out.
My BMW agent is totally confused as to the cause, has anyone had a similar experience?