FoMoCoSHO - Thanks very much for the note you sent.
To provide everyone an update on the situation, I have concluded that the knocking noise I am hearing was not due to the water pump, but actually due to the cam phasers knocking at idle. The fact that the noise only occurs once the engine starts to warm up (and the oil thins out) and also only at low RPMs (under around 1,000 rpm), are both very good indicators that the sound is actually from the cam phaser lock pin making noise while seated. The noise goes away when slightly off idle.
After watching and listening to lots of YouTube videos of cam phaser issues on different Ford engines, it's now fairly clear to me that this is the sound I've been hearing. What I can't explain is why the Ford techs at my local dealer were unable to make that diagnosis since this is something they probably see (and hear) on a regular basis.