[moodman]

If no gas is flowing, then it is impossible for a noise to be generated. So, in my thinking, the flow can be only one of three possible directions.
1. From the tank to an appliance. You didn't say, but is this sound ONLY when an appliance is working? This is important, because one of two things can be happening. Either the appliance leaks slightly when off, or gas is flowing towards the appliance (when off) and leaking before it gets to the appliance.
2. Fro the tank to the other tank. You also didn't say if you turned off the other tank's valve. If you did, and the noise is still there, and you have eliminated possibilities per #1 above, there is a leak between the two tanks.
3. The flow is leaking at the tank. It could be in the regulator itself or a connection near the tank.

I suggest getting a bottle of soap bubbles like kids use or you can make your own soap solution. Put it on ALL connection points and observe if there are bubbles.

Good luck, but you MUST fix this! Its potentially dangerous.