It could be the pressure relief valve that is passing a small quantity of water. The outlet of this valve should be piped through the wall of your house. Find the end of the outlet pipe & see if it seems wet or is actually dripping. I checked mine by attaching a plasic bag for a couple of days (it filled with about half a cup of water in that time). If it is passing, you need to replace it. The problem I've had is that it was replaced about three months ago - everything was OK but has started leaking again & am not sure what is causing this.
Hope this helps.
Brian Ninness
September 2010