Halfwright:
574rvman has problems... and you, your making fun, shame shame.
BUT......
I must confess, I had the same thought.
Answers provided by others seem plausible to me.
Another thought ......... slowly change pressure to RV (pressure regulator at input) and see if influences humming, and I don't mean tune one hears or doesn't, just loudness (oh God, help me).
modified my response... after seeing that 574rvman identified Pressure regulated as culprit. I should have seen that!