I have a knocking sound in the pipes when certain faucets or appliances are turned on. It happens with 2 out of 3 toilets, one shower, the washer. This is while the water is running (I'm assuming not a water hammer). When the washer is started, I can turn on a faucet in the downstairs bathroom adjacent to the laundry room and it will stop. We have turned off the main and drained the system (including the sprinkler system and outside faucets) and this worked for one day. Any ideas? When we drained the system should we have left the faucets open when we turned the system back on? Any help will be appreciated.