In cleaning dirty gutters it is immediately apparent that the smell generated by the standing stagnant water along with decaying leaves can be offensive. Depending on the types of leaves that are rotting in the gutters the smell can be a lot worse. Oak leaves and acorns have a very pungent smell. When covered with water from the gutter being clogged there is not a lot exposed to the air, but as the dry out or as you pull them out of the gutters with contact with the air it gets much worse.
These smells can possibly get into the house by airflow from the roof vents or the stack vents. How ever these smell get in the house they can be distinctive if you have cleaned gutters. The out gutter smell can get into the house if your plumbing vent pipes dry out. The trap filled with water creates a barrier to the smell, but without water in the trap the free flow of outside air and smell can occure. Many times the bath room vent stack can be close to the rain gutter line. So the chance for cross over certainly exists. Eves vents installed inside of the sofit can be another place that the smell gets carried up into the attic and then possibly down into the house.
Other Sources of Smells
There are other things that can be confused with gutter smells. A roof that remains wet and gets moldy can produce the same type of musty smell from the mold on the roof and then get inside. Unfortunately there is the other possibly that a mouse can get into the roof (you would be surprised where they can climb to) and die there. The smell from a dead mouse can also be similar but different from what you would get with dirty gutters
The Solution to Gutter Smells
There really is no hard and fast solution other that the process of elimination. Since the gutter smell usually comes and goes keeping track of when it is worse and the weather will help you figure it out. What room the smell is worse in also helps when you check that room and any vents that may close to that room. Cleaning the gutters may solve it and if they have a lot of standing water and rotting leaves taking them out will certainly give you a sample of the smell. Having the roof cleaned of mold, mildew and moss can be another solution. In trouble shooting the smell going into the attic near the source should help to see if it is inside the attic or comeing in from out side.