Shiloh runs and hides when she hears someone knock or hears the doorbell. She recovers very quickly though and doesn't shake or anything.

While I am against medicating animals for behavioral purposes, sometimes I think it is just necessary. Maybe your vet can prescribe something that will calm his nerves for a few weeks while you desensitize him to the doorbell sound?

Or maybe you can try Rescue Remedy in his water? Rescue Remedy is all natural.