Thanks Petal...I will ask my OH to do that tomorrow. I don't touch the Sky box! I will also call Sky. It is the most peculiar thing, and you could bet your last penny that if I called out a Sky engineer, it wouldn't be humming when he turned up!  

Thanks to everyone who has posted....I will show all the posts to my OH tomorrow to see if anything rings a bell with him. It isn't actually bothering him, because he can't hear it!