Last week, I heard a squeal coming from the pipe leading into the house over to the hot water heater, but not the heater itself. I'm not sure of the exact point. The squeal starts whenever any water in the house is turned on and stops immediately when turned off. This continued while I was out of town.
When I came back five days later, the squeal quieted, then went away. About two days later, it seems that any water faucet that is turned on starts with a slight 'rush' then returns to normal.
What could this be?