I was getting bubbles in the reservoir every time I shut off the engine. I epoxied the ps pump npt tapped inlet and the bubbles stopped.
Here is a video of the sound I'm getting. Is this similar to your's Matt?
I have an explorer 5.0 and used the Dorman 300-301 which is 5-5/16 OD. My other though is where I tapped the pump 3/4 Npt is weeping slightly. I wonder if it could be sucking air when rev'ed up. The rattle seems to start about 1500rpm.
I used this same approach with a brand new acdelco pump for a 2017 duramax, and everything seems to work fine, but when i increase the rpm i get a loud rattle.
My setup used a 3/4" MNPT x -10 AN straight fitting and then a 150° Long sweep an fitting Vibrant Performance Push-On Hose End...