Water spray noise when using the on/off button on the shower wand

[On The Road Again]

I have noticed of late that when I use the off button on the shower wand while showering, I hear what sounds like a spray of water immediately when the pump kicks on. It sounds like it is coming from behind the wall. I tried it with just the cold water on and it does not do it. It only does it when the hot water valve is on. It does not do it if I simply use the hot and cold water valves to temporarily turn the water off. I am looking for suggestions before I have to remove everything from the pass through and the wall separating it from the underbelly. Could it be the shower vacuum breaker?