[Porsche-O-Phile]

Although I've never done banner-towing personally, my understanding is that they fly in the "region of reversed command". To explain this simply, it requires more power to fly more slowly in this region due to the aerodynamics involved (induced drag at high angles of attack).

If you look at most of the banner-tow guys, they're usually operating with all the drag devices deployed (flaps, gear, etc.) in order to put the aircraft in slow flight. This requires lots of power and is therefore noisy.