Joe Bfstplk
Legend!
I bet it was the plastic rack being thinner, lighter, and more prone to resonances. The wood is likely a lot stiffer and heavier, so it does not sympathetically hum along....It might had been less noisy in the gigging rack actually. My unit was okay as long it was in a wodden rack. It seems the vibrations got eaten up by the wood construction and the rubber feet.
Then I took a plastic rack and the noise got very noticeable. It's not about the air stream, it's about vibrations.