Clicking sound from gas tank area when brakes are pressed or car turned on or off

Hello All,
A have a 2008 Prius new to me. It makes a clicking sound from seemingly on top of the gas tank. at first I thought it had something to do with the gas tank (I have had to replace 2 of my 2009 prius gas tanks). However, I eventually noticed that the clicking sounds occur at key near car before ignition on, when sitting stopped with brake pressed, and after ignition off for up to several minutes. I have replaced about six actuators and this isn't that. Eventually after several weeks of driving it produced a C1391 loss of pressure code. When I cleared it with techstream the clicking sounds got a lot quieter but still there. It almost seems as if there is some sort of brake pressure relief valve on top of or near the gas tank but I have no idea if this is even a thing.
Help. An no this isn't actuator noise from up front. I have the back panels and seats removed as I had to replace the hybrid battery after purchase land can hear the noise seemingly on top of the gas tank. I would thing some sort of evaporative purge noise from something failing (which would have been annoying as I discarded the old tanks after replacement). It is clearly linked to brake pedal pressure when stopped as at a light. No sound during mnormal driving. And lifting foot to barely stopping does lessen or stop the sound.

 

Yes, got the letter wrong. I had a hard time finding out how to even post the thread. No edit button that I see. It was in fact a C1391. Edit button here naturally :}