I hear a thumping in the wall whenever the second floor bathroom shower, toilet, or sink are used (sound is similar to if you let the bottom of your fist fall against a sheetrock wall). It is 'almost' at a regular interval. I also don't recall hearing it in the summertime, but have only been in this house for a year. It is related to the drain, because when I put my ear up to the wall where the sound is coming from, I can hear the sound water rushing down the PVC very clearly. Once the water is off, the sound stops. Does anyone know what might be the cause of the sound?