I have a situation that I am not sure if it’s a plumbing problem or possibly an air conditioner problem. Here it is.

Our house is 4 years old. Since moving in we have had an awful smell (perhaps a “dead” smell, I’m not sure what that smells like) in our down stairs guest room and my son’s room upstairs. The rooms are not on top of each other. They are next to each other. I can not determine where it is coming from. It only appears every now and then. It is in the guest room through out the year but in my son’s room only in the late Spring/summer and isn’t necessarily in the guest room at the same time. When it is in my son’s room it’s far stronger than when it’s downstairs. I have had the carpet professionally cleaned and deorderized and have had our duct work video inspected. Nothing was found and the problem persists. There is a bathroom below my son’s room and next to the guest room. I have never noticed the bathroom smelling. Could this be a sewer gas problem but not be in the bathroom? We don’t use the bathroom downstairs very often. I have started pouring water down the drains but it still comes (I’ve only been doing it a couple of weeks). I thought I should send some cleaner down the drains as well but wasn’t sure what to use. I should also mention, we have city sewer but we had a temporary septic tank for two months when we moved in. Because the smell is not persistent, it’s hard to have someone come over and smell it. It lingers only for 20 –30 minutes downstairs. I have noticed the two times it was upstairs that it stayed for over an hour. It came two days in a row upstairs. We have run our air conditioner once this year and the air conditioning unit sits outside our guest bedroom. Any thoughts would be greatly appreciated. I’m getting very frustrated. Thank you so very much.