Below is a picture of the lighting instructions for your water heater taken from Rheem's website. Make sure to follow the lighting instructions carefully and do not skip any steps. This same instruction set should be available as a sticker on your unit.
It looks like you do in fact have to depress the knob in order to turn it so avoid using force.
If you are ...
Try running the water for several minutes, both hot and cold sides, at the other fixtures too. Do they behave similarly? Do the symptoms diminish after a minute or two?
When a water system is opened, especially for something like a heater replacement, air gets into the pipes. Eventually the air finds its way out through the various faucets. As bubbles of air ...
An exterior garden hose spigot was on, leaking, and whistling. Turning off the outside spigot stopped the whistling in the Rinnai. The water heater must have acted as an amplifier for the sound travelling through the pipes of the house.
Since you didn't say it, and it matters - first, shut off the power or fuel to both heaters, so they are not dry-fired, which will quickly destroy them. This is very important any time a water heater is not full of water.
To flush the second tank, if they are connected in series, consider the first tank as if it was a pipe.
So, shut off supply, open hot taps,...
Depending on your boiler, you can at least see the DHW called temperature on your boiler display. I have the Smart 30 water heater coupled with a Triangle Tube Prestige Solo boiler, and on its display, under the various info items, I can see the DHW temperature. I'm guessing that will be the closest you can get with this specific setup.