This is a very common problem that happens when you use hot water. To test and be sure that it is not a leak you can do the following:
1. before you go to work in the morning turn your water heater thermostat all the way down so that the water will get cold in the tank.
2. when you get home run the hot water in the kitchen sink and make sure that the water is not hot.
3. Now run the hot water in the master sink (should be cold or just warm water coming out) and listen for the "dripping sound". If you don't hear it you know that the problem is just the plastic drain lines squeeking against the wood studs in the wall as they expand from the hot water running thru them. I do not know of any way to fix this without cutting open the walls open.
Don't forget to turn the temperature on the water heater back to its original setting. It should take less than an hour for the water to come back up to temperature.