CHICAGO (CBS)– Chicagoans have been startled by loud booms and cracking sounds, and they’re not sure where the noise is coming from.

One possibility is a phenomenon called “frost-quake,” which happens in cold weather.

A contractor spoke with CBS 2 and said this happens a lot. He said when the ground gets saturated with water and ice, it can expand and crack. this makes a loud booming sound.

Chicago’s Building Department said those loud sounds also can be made by metal or wood in your home, expanding and contracting, because of the cold.

The noise could be caused by house frames, rooftop, floorboards and even air pockets inside your house.

It appears to be a natural occurrence during cold weather.