In the past few weeks I have noticed a problem with the cold water for my tub/shower. When the cold water is first turned on, there is a noise and shaking coming from the water pipe (inside the wall). This doesn’t happen every time, only when the water has been recently used (someone else just took a shower, etc.) If the water hasn’t been used in several hours there is no problem. What is this? Air in the pipes? If so how does it get there? Everything seems to work fine, water pressure the same, etc. and no sign of leaks anywhere.