Tom I don't have any evidence to the contrary but I know what the beeping noise is without a doubt.  The hissing on the other hand could be a lot of things.  My vote is for the beep.,(reply>), you obviously haven't had much construction going on around your home. When those beeps are constant, their maddening. Also, when many different trucks are backing up at the same time, the beeping doesn't help at all. Occasionally people just stop and panick as beeps are coming from many different directions, or just get so used to it they pay no attention at all.