A month ago I was home alone and it was quiet. I heard the dreaded sound. It was emanating from the wall near the water heater and shut-off valve in the garage. I opened up the meter outside and sure enough, it was moving a tenth of a gallon every three minutes. I confirmed it was not from the outside to the shut-off valve. I messed around with everything and couldn't pinpoint the leak. A plumber came in and showed me that the valve on one of the upstairs toilets had to be replaced as it was constantly filling the tank with water. I replaced it myself and all seemed to be good to go. Well, I heard the sound again today. After turning off each valve at every water source one by one, the sounded persisted. This included the toilets (starting with the troublesome one), sinks (hot and cold), washer, and refrigerator (which wasn't in use, but I made sure it was tight). My question is this: if one of the toilets were leaking, would turning off the water supplies at the toilets make the leaking sound that I hear near the shut-off valve stop? Secondly, we have a water filtration system that one of the major purified water firms installed. Would it cause water usage even when it's not serving water?