My water pipes have recently developed a hum or a horn noise in the pipes after using the water. It sounds like a ship in the night honking it's horn. Sounds quiet and far away. This is new. Can you tell me what might be the cause of this?

Could be a loose washer in the main shut off valve or a defective pressure reducing valve. Most likely a the pressure reducing valve if you have one.

Feel the pipes in that area, you should be able to feel the vibration in the pipe.

Edit, Just noticed that you said after
using the water

If that means after you close a faucet, it will be the PRV valve.