RATM909 - hi, I'm not a plumber but I've found and fixed broken pipes before. If you want some immediate peace of mind, I'd recommend you check your water meter for signs that water is constantly flowing. If all of your faucets are turned off, you should see no movement in your water meter. If you see movement and you know nobody's turned on any faucets, flushed any toilets, etc., then you may have a problem you'll want fixed soonest. The sound you're hearing may be a sign of a cracked/split pipe in a wall. Also, even though a hose spigot/bibb may be listed as "frost-free" it's always best to disconnect the hose from the spigot during the winter months when water lines can freeze, just to help prevent a pipe cracking/splitting.
If you wait until your first water bill comes in, the water supplier may not be willing to write off the cost of the "lost" water, especially if they only bill you bi-monthly or quarterly. They may be willing to come out and investigate the noise source, but if it's traced to inside the home they may not be willing to fix it. In my case, the water line outside the house was broken and they located & fixed it even though they hadn't been working on the pipes recently. (The break was located between the street and the water meter.) The water company didn't bill me for the several thousand gallons of "lost" water - and that was lost in only about 10 days!
As you trace your pipes, you should note where your main water shut-off valve and any other shut-off valves are at, to assist in future repairs. I hope this helps some - good luck! Jim D/West Point, VA