2010 Mustang Water in Dash sound?
 

Hey guys, first real post here. I figured I could go to a Mustang group for help. Recently I've run into trouble with my Mustang, and never had any issues before. When I start the car, I hear water sound almost like it's moving around as soon as I turn the key, and I could hear the water moving when I turned corners. It had been raining pretty heavy here for the past week, so I attributed it to that. Two days later, I got in my car and it smelled AWFUL, and there was a puddle of water on my passenger side. Did some digging, found out my cowl vents could be blocked on the passenger side. Well, I went it, unplugged the holes, got the gunk out, and water came gushing out of the passenger side. I thought I fixed the problem as the interior of my car is completely dry after I toweled and blow-dried it, but I'm still hearing the water noise, but now, I only hear it when I start the car, and it only lasts about a minute, and I no longer hear it when I turn corners. I heard it could be my heater core, so I checked my coolant and it was low, however, my heat still works great and I haven't had any issues at all with the way the car runs - I drove about two hours on the highway and the car never showed any signs of overheating, anything. No smoke out of the exhaust either, and the coolant level has remained very steady since. The water noise is also less prominent since I added coolant. Any idea what could be causing the water noise? Is it just air trapped in the system? I thought air in the system made the coolant appear to be higher than it was...could I maybe need a new reservoir cap? I still have the original from when I got the car.

It's a 2010 V6, btw. I am the only owner.