Strange. I had a similar thing years ago. When the pump would repressureize the system from any bathroom use the pipes would howl like a train whistle. No matter what I did or checked, as the months went by it would get louder (I mean it sounded like a train coming through the house). Eventually it slowly started to go away and never came back. I suspect it was a calcium deposit of just the right shape that continued to change shape until it wasn't a "whistle" any more.