My master bedroom (bathroom?) has this rotten smell that I can't trace to anything. We have a septic tank but all the other bathrooms are fine. I checked the attic and found no dead animal or anything. The smell is sometimes strong sometimes weak. It is not obvious that the smell is from a specific place, like the toilet, sink etc. When I fluch the toilet I hear some rubling sound. Could it have anything to do with that? Any insights will be greatly appreciated.

Do you have forced air heating? When the furnace kicks on it stirs up the air and may dissipate the odor. After it has been off for a while, it builds up. You can isolate things by taping over a drain. If the smell comes when a drain is taped over, it is coming from someplace else. Note, tape over the overflow as well as the drain. Check the drip pan under the refrigerator. You can have a bad wax ring seal on a toilet. Taping around the base may stop it long enough to tell.

Above from a little utility that saved it from a similar question last night. The rumbling makes it even more likely it is the toilet.

Thanks a lot for the suggestions. Let me do a little leg work to eliminate some causes. What is an "overflow"?

"What is an "overflow"?"
When Labman told you to tape over the overflow he meant the tub waste and overflow in your bath tub. That's the chrome plate with the little lever just below the tub spout. Good luck, Tom

I think I might have found the culprit. It's the jacuzzi bathtub that we never use. Now I was not sure which part of the bath tub that's causing the problem. There are some failed chaulking between the bathtub step and floor that smells bad. The bathtub drain smells terrible when we use the shower (not at the same place as the tub). I follow the advice discussed at this forum and did the bleach and hot water exercise to the drain. I also re-chauked the bathtub. Now the smell is much better. I'll report whether I have fixed the problem. Thanks a lot for the help.

"When I fluch the toilet I hear some rubling sound.".. fluching is bad for your toilet, try flushing. Hope this helps Jonas,,