Water leak in walls - copper pipes, I heard the hissing sound in my garage near the cold pipe line expose?

now it‘s getting louder, more like a faucet slowly running. I do not see any signs of water damage around and near the water lines. Could this be the line between the meter and my shut-off valve that leads to the house?

I would suspect the line to the outside tap has frozen and split.

It very well could be because copper pipe carries sound very well. If the ground is frozen it may or may not come to the surface. Turn the water off at the meter and if the hissing stops, and there is no signs of water in the house, then you wil know.