I have a grinder pump in my downstairs bathroom. The whole downstairs is below ground. Kitchen sink, washing machine, shower, bathroom sink and toilet are pumped into my septic. Every once in awhile there is a septic odor in the bathroom downstairs when my daughter runs the shower for twenty minutes; and when you first run the water "upstairs" a septic odor comes up from the the bathroom sink drain for just a few seconds. I think it might be venting. Septic was pumped 2 1/2 years ago.