Pipe noise upstairs bathroom when running only warm water
I have a 6 year old 2 story house on a slab. The bathroom on the 2nd floor has one of those single handle tub and shower fixture. When I turn the single lever all the way to hot or all the way to cold, there is no banging noise. When I put the lever to the middle to mix the hot and cold water, the pipe starts banging.
This weekend I cut the drywall behind the bathtub to see what's going on, and noticed the cold water pipe seems to be banging and vibrating only when I put the lever exactly in the middle position. Again, no problem in only hot or only cold position.
Also the spout diverter and the pull pin has competely fallen out. I tried putting it back in but it just falls out when you pull on the pin.
This faucet is the only one in the entire house (including toilets) that is making noise.
Since the banging only happens in the middle warm position, could it be the faucet itself that is causing this. I want to see what you guys think before I spend money replacing the faucet or doing something unnecessary.