Well of course,they certainly should leave their houses and go out into the garden,in fact they should do it whenever they feel like it,and maybe in so doing they might learn to be tolerant of the percieved noise.i know that i,for one,have learned to be tolerant of the considerable noise around where i live.
i strongly doubt,however,that they will be in their gardens long enough for the noise to start causing medical conditions,and if it does,then perhaps they are of such feeble constitutions that it would be better in any case for them to stay "huddled up" indoors.
PS:in heathrow's case,i think the chances of people WANTING to spend large amounts of time in their gardens is a bit far-fetched, unless you're talking about asylum seekers from Iceland living in Hounslow!