Just went thru this this past summer. Definitely the plastic poppet check valves. Yes, it is a pita to access the back side of the water heater. Fortunately I had a friend who helped me. We pulled one which was still intact, buttoned it up and had same issue, damn! Took the other connection apart and found the check fell apart blocking flow and causing squealing.
You don't need them, don't think any other manufacture installs them. I had installed a "Camco" bypass kit when new and I prefer to blow out and add antifreeze. I would NEVER install them.
Tara is our Border Collie daughter.
2014 Winnegabo Forza 34T
Dragging a 2015 Cadillac SRX, very nicely!