I have hada 55 reg 2.0TDCi. for about 2 years and I like it alot. I have noticed a low level hum which occurs for about 15 seconds after the turbo whine stops when i turn off the engine. This does not worry me. But the hum also occurs for 15/20 seconds when the car has been parked and locked for several hrs. Eg parked and locked at 2pm afternoon. Hum occurred at 0.15 am and 6.45 am this morning. (It may have happenned more times but I happened to be awake at these times and the house windows are open this hot weather and night is very quiet so it is noticeable). Can anybody help, what is the hum, is it normal?