Other then something is rotten or died…the number one cause of a bad smell in your house comes from problems with poorly vented drain lines.
Drain line vents are the vertical iron or plastic pipes that run from the drain through the roof. When functioning properly, they allow air into the plumbing so pressure inside the drains do not form an air lock that stop waste water from free movement.
The first sign that you have trouble is a noxious odor. Traps inside your sink plumbing contain a little bit of water that helps keep the sewer gas out. If the vent is clogged or obstructed, a sewer smell is produced. It may require a snake to remove the clog, or for the plumber to flush out the vent with a hose.
If you are hearing strange sounds from your drains, especially when they are not being used, it’s possible that air pressure is not escaping as it should to properly vent the pipe.
If you are finding that a single flush does not clear the toilet bowl, the vents may be blocked and not enough pressure is produced to encourage the flush.
Another indication that your venting may be clogged is: air bubbles in your toilet bowl. This is especially true if your washing machine is located near the toilet. The force of a draining washer will push air bubbles into the toilet when the air pressure is not properly vented.