That could be many things, the most likely one would be a hole in a intake line, It would cause a hum because of the plastic molding and the way the air hits it.
That's called the engine, it powers the car, and eats gasoline, unless the car is turned off, the engine will persist in making noise. It is a very common problem with all cars that have gas engines.
A number of things could cause this, but most likely you're experiencing a wheel bearing going out or a bad tire. Check those first. Otherwise, it's the engine.
One key reason could be that your silencer pipe is leaky. Check if the silencer pipe is broken. Replacing that could reduce the noise in your car. Leaky silencer pipe can create really loud humming noise. Your silencer could have been broken while driving through a rugged terrain. In countries with rough terrain, cases of broken silencer pipes are high.
If your Dodge Caravan is making these types of noises then your transmission could be going out. You will need to have this repaired or fixed as soon as possible.
