Hearing a strange sound in your home is always disconcerting, especially at night or if you’re in the basement. Some sounds are natural. Homes are built of many different materials that make noises as they expand and contract.

If your home is making noises louder than a knuckle popping, it could be a sign that there’s a problem that needs immediate attention. Here are the sounds you don’t want to hear.

Hissing

A possible gas leak could be indicated by a hissing sound. This is something you definitely don’t want to mess around with. Don’t even mess around with trying to shut it off. Get out of the house and call a professional to come out ASAP.

Clanking

When you turn on the heat in the fall, it’s very likely that it will make a little noise. If you hear knocking or clanking, it could be a sign that the circulator pump is about to fail. Call an HVAC service technician and get them out there ASAP.

Scratching

If you hear a strange scratching sound coming from behind the walls, it’s a good indication that you’ve got critters such as mice, squirrels or raccoons taking up residence. Set traps or call a specialist ASAP. They can spread disease and cause damage to the home if not removed.

Bubbling

Any time the water heater makes a weird sound is an indication that it needs attention. A bubbling or cracking sound usually means that you’ve got sediment built up in the tank. You should flush the tank at least once a year. This gets rid of the sediment and extends the life of your water heater.

Whistling

No, your heater doesn’t think you look great. That whistling sound means your heater is working too hard to pull air in. Replace your filters every three months.

Running water when no one’s running water

This could indicate a busted pipe. Shut off the main; if the sound goes away, you’ve got a leak. Call a plumber. A leak can cause damage and promote the growth of mold.