I've read other articles where they say the problem is likely the was ring.
You have a ruptured wax seal that's allowingsewer gas,( methane) to enter your home. Pull the toilet and replace the wax seal and here's how to do it.
Turn off the water to the toilet at the water supply shutoff. This is usually found on the wall or floor near the toilet. If there is no localized shutoff, use the main water shutoff for the structure.
Flush the toilet a couple of times to drain most of the water.
Disconnect the water supply tube from the bottom of the toilet, using a large pair of adjustable pliers. (Have a bucket and towels ready as some water may drain from the tank.)
Find the toilet mounting bolts on each side of the toilet base. These generally have decorative plastic or porcelain covers that can either be pulled off by hand or gently pried up with a flat screwdriver.
Remove the nuts using an appropriately sized wrench or an adjustable wrench.
Lift the toilet straight up off the mounting bolts and move it out of the way. (Toilets are fairly heavy. It might be a good idea to have a friend help with the moving.) Also be aware that there's still a gallon of water left in the trap.
Examine the closet flange on the floor. You should find a yellowish wax ring about 4 inches in diameter - some brands have a plastic base that will have model information on it.
. Occasionally the ring will stick to the toilet base. If it does, tip the toilet over and pull the wax ring loose from the base. (Be ready with towels or rags.)
Place the new ring around the horn in the base with the flat side down. Set the toilet back on the closet flange.
Seal the toilet onto the new ring. This is most easily accomplished by sitting on the toilet and very gently rocking in all directions. This will flatten and conform the wax to the base of the toilet.
Replace and tighten the mounting bolts and decorative caps.
Turn on the water and allow the toilet to fill. Flush and check for leaks. Good luck, Tom