Pipe noise.. Sign of problems?

[Kevz]

Last night, sounded like a motor of some type was running in our basement. I ran down to see what the heck the noise was. Only thing I could figure was the pipes. Shut off the main and it stopped. Went outside and noticed the city was opening the fire hydrants on all the streets for a while on all the blocks.

Is this normal, or a sign of a problem in the system somewhere? Sound was amazingly loud. Was even making some of the air ducts vibrate.

Thanks

[plumber1]

First thought that comes to mind that there is water back-flowing to the outside. And that there may be a ballcock or a faucet that is letting ever so slightest amount of air to enter and allow the vacuum to pull water back to the city main.