Sound is likely a vacuum pump.
Water and maybe others can freeze and boil when under vacuum.
Is why you need to warm AC systems trying to draw moisture out using a vac pump.
In cool weather is very hard to impossible to draw water out from acc/drier and AC oils. Old mineral oil for R12 was bad but oils for R134 are worse. They love water about same as brake fluid.
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)
The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)