I asked exactly the same question when I first bought my Mark 3 Mondeo!
There's a small fan which blows air over a temperature sensor to measure the ambient temperature. The noise is probably that. It starts when you turn the ignition on and runs down when you turn it off.
It's quite normal - nothing to worry about!
Edit..................I've even found my original thread!
www.honestjohn.co.uk/forum/post/index.htm?t=100771
Edited by Galaxy on 16/03/2016 at 10:32
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