I have a smelly bathroom that just started several days ago. It seems to go away at night but comes back during the day. We had the septic tank cleaned out but it was found not to be at normal levels. It also didn't solve the problem. The service man suggested that the smell could be because the vent is close to the window of that bathroom and with the warm weather we've been having lately, it has caused the smells to be stronger and comes in. But this has never happened in the two years we have been here. We also suspect it may be something in the drain of this bathroom because we have noticed it hasn't drained well lately. We still wonder why the smell comes and goes even when that bathroom isn't being used. We've also noticed the smell gets stronger when we use any water source in the house so that is why we ordered to have the septic tank cleaned. Any suggestions of what the problem may be, or what else we might try?

Your vent may be clogged a little.