Ever noticed those knocking drain noises when hot water drains out of your sink or bathtub? It's the drain expanding. That's it, that's all. No big deal. It sounds like a leak, but it's not. Here's a video demonstrating this: https://youtu.be/c9k8PiJRTHI. Nevermind the crazy thumbnail.

Author: Reuben Saltzman, Structure Tech Home Inspections