I need answers to 3 questions
1. why is a "p" trap designed to always hold water
2. I need to know 2 reasons water continues to run in a toilet tank
3. can a faucet washer cause a faucet to vibrate and make noise

1) The Trap is to keep sewer gas back into house. A water block.
2) Leaks will cause toilet to keep running or on and off.
3) Water can squeal when going through small space.(leak in a washer, and could vibrate).