The water pipe in my bathroom rattles randomly without a valve being turned on. The duration can be from fifteen to thirty seconds. I can turn the cold water tap on and the water pulses until I turn the tap on to a full flow. If I flush the commode it will stop, and if I turn the hot water on it will stop the vibrating but not as quickly as turning on the cold water. The house is six years old and built on a slab so main valve is located in the house near the tankless waterheater.

Next time you hear the noise remove the top to the toilet tank, reach in and lift the float. If the noise stops, you have a faulty fill valve and a leaking flush valve. Repair or replace the fill valve. Replace the flush valve.

In addition to Harolds check the water level in the tank, It should be n1/2 under the top of the white overflow tube when the fill tube shuts off. Good luck, Tom