Q: I put in new plumbing for a new tub in a remodel and every time that I turn on both faucets, they make a loud noise (like air in the line or something), but when I only turn on one faucet, it doesn't happen. What could be the cause?
A: First, try removing the handles and stems. If the washer is floating and/or the seat is loose, tighten everything back up. This is a rare situation, but it does happen.
Another possibility is that the water pressure is too high. In this case you need to call a plumber to adjust it, or troubleshoot to see what could be the cause.