Pipe noise.. Sign of problems?

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

noise

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.