Water sound while driving, especially at low speed

I recently discover the sound of water coming from outside of my car. It sounds like water going down the drain. It is very audible when driving in enclosed areas, like a very narrow alley or neighborhood where there are cars on both side. It's especially loud when the car is moving at a very low speed, like when it's rolling without the food on the pedal. But it's barely audible when in open areas. It can only be heard when the driver windows are down.

At first, I thought it was the A/C, so I turned it off but it was still making that sound. Anyone know what this may be? The fluid levels are all good.