unexplained phenomenon

Today, strange sound wake me up. I put some light on the room, and find out, that source of this sound was the bicycle tire loosing air. Tire when loosing pressure, making specific sound, because of changing the volume and shape. The thing is that bike was sitting untouched for like couple days, and pressure was good. Air loose was suddenly, 2 mins top. How this is possible ?
That's not all, I repressurized the tire and it holds the air ! There is no hole, and valve is fine.