Are u seeing this on the highway or an interstate? Or do live by mountains? It might be for the truck drivers benefit. Most semi's has a breaking system called a "jake break" It when they use this when they are likey to be in a mountain area and they need to go slow, so theyu use this. If they do not, they run the risk of burning up their break pads, and plus its more breaking power, cause some trucks might be carrying 80,000 lbs or close to it. So when the trucker use this, it is very loud so if u might live in a city that has a noise ordinance and they want the noise at a certain level. U have probably already heard it but u didn't know what it is.
at 2:36 AM on Dec. 28, 2010