This sounds like redirected aggression to me. Redirected aggression is a form of aggression in cats that happens when a cat is aroused by a certain stimulus, but is unable to direct their aggression on that stimulus.
The stimulus in this case is the sound of the toilet valve, that leads to a heightened level of anxiety in your cat.

Do you happen to know when the valve makes the noise each time? If there is a way to separate the dog and cat when the noise is happening (or stopping the noise entirely), that would be best.
Also, to calm your cat, put him in a dark and quiet room for him to calm down in or try to avoid him (and/or keep the dog away from him) while he's aroused by the stimulus. This might be hard if it happens at random times.

**hugs**