1.5hp submersible pump, about three years old. Water well serves home and lawn irrigation. When pump is running, it makes a howling noise. At least it sounds like it is coming from down in the well. As pressure builds up in the tank, the pitch of the noise rises. Once pump shuts off, noise stops even when water continues to be used. Heard the noise the first time about a week ago when irrigation system was on. I shut off the irrigation system, and opened a faucet to drain the tank enough to start the pump again. Howling wasn't there. Ran the pump several times with irrigation system off -- no noise. Ran the pump a few times with the irrigation system on -- noise every time.

Then today when I went out to the pump house, the pump was running due to the water softener regenerating. No sprinkler zones on. Pump was howling again. Drained tank from faucet again to get pump started. Howling noise started again.

Otherwise, pump seems to be working correctly. It fills up the pressure tank in the same amount of time. It keeps up with the sprinkler system like it always has. Water pressure seems normal.

Any ideas?