Thanks for the reply and info. Well I am puzzled and yes I will go back to the garage and ask if all else fails. I guess I am more curious than worried! Someone who has a petrol Scenic has indicated their car does the same thing. OK well that being the case it knocks the
lubricating pump theory that it goes on working sometime after the engine has been cut off. I tried it again making sure the AC and cab fan was off, noise still there as before.
I should mention that the Scenic is LHD as I live in France though I would have thought that there would be little difference between mine and models sold in the UK.
If I sit in my other car, an oldish Megane, with the engine off and with the radio on 198 LW, Radio 4 is weak reception here, and switch on the lights the tone appears as interference but no noises under the bonnet. If I turn off the lights then the tone continues, audible only on the radio, for about 60 seconds and then stops. Similar thing happens with the Scenic except the tone stops as soon as I turn off the lights. In both cases the engine had not been run for at least an hour. I suspect that this has nothing to do with the noise heard under the bonnet and must therefore be due to something else, just coincidence that the frequency sounds the same and we are dealing with two different things.
Having run the engine again on the Megane I think the noise from under the bonnet after the engine has been stopped is a fuel pump of some sort. A black plastic object mounted on the bulhead near the top right, looking towards the rear, and has two black fuel type pipes going to it. The noise in the Scenic is from the same area. Also in the Megane there is another buzzing noise towards the front left hand side of the engine and low down, again looking towards the rear, which is only audible if the ignition switch is turned on but the engine not started. Cannot do this with the Scenic as there is no ignition switch other than the motor start button.
I think I hate Smart Alec cars almost as much as I hate computers. Never had these things going on with my first car, an Austin 7 (Ruby), once got nicked for leaving the starting handle in, so even more puzzled now...............JR