Sounds like a venting problem. But, I am not sure what you mean by the water inlet valve. Does water run in the guest toilet when you flush the master toilet? That would indicate a blockage in the vent pipe. You may need to get a plumber to go on the roof to check for a blockage in the vent pipe. But, I would start by replacing the flapper or sealing gasket in the guest toilet tank and see if it fixes the problem. That is a simple do it yourself task. You may also try shutting the toilet supply valve to the guest toilet (pull it out), empty the guest toilet by flushing it, and then see if you still get the noise.
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