Every night for the last week or so, the water heater here starts making a loud humming/buzzing noise. Happens every night at 9 PM -- you could set your watch to it. This doesn't seem to correspond to anything going on with water usage here. Drained and flushed it a few days ago -- no change. The vibration seems to be concentrated on or near the cold water inlet pipe -- that is, if you touch it, the vibration is real strong compared to the hot pipe. Heater is 10 years old. Any ideas?