[Richard]

Hmmmmmm, sound can travel. Bubble solution is your friend, much more reliable than my old ears.

I would slip between the tag and drive wheel and listen. You could have an air leak back there and the sound is traveling. There are air lines that run the center of the coach, but unless you have rubbed a hole in one, over the black tank is an unlikely place. Not saying impossible, but not the likely source.

Five minute to 90 psi from 0 doesn't sound too bad. And if it built 90 then it can't be too damaged. That oil left the system because the pump ran continously for an extended period. It became very hot during that time and two things happened. You boiled the oil, and you tripped the thermal protection.

I would change the oil in the pump completely before putting it back in service. The oil that remained is likely compromised.