OK, I'm going to take one more stab at this. If all else is not the issue, then the pump impeller may be bad. I am not sure if you can disassemble it and replace or you have to replace the pump. Most I've seen are not serviceable and must be replaced.
It sounds like the pump is not drawing directly at the inlet on the pump housing, in which case, the impeller is not sealing enough to create a suction. Could be worn out or even dry rotted.
Robert & Diana McNeal
2019 Montana Legacy 3791RD 20th Anniversary Edition
2014 F350 4x4 6.7L SRW