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Probably the power transformer core laminations are loose and are humming to the line frequency.
It might be advisable to contact GW Instek and let them know that the transformer is humming (if you determine that is what is actually making the noise), and they might send you a new transformer.
Depending on how the transformer is made, you can sometimes wrap the core with electrical tape to keep the laminations from vibrating. I have seen where some people re-varnish or paint the edges of the core laminations and achieve the same thing. A coating of epoxy will instaed of varnish / paint would work, as well.
Will suggest to seek help from distributor you bought it from.
If you're having trouble locating the source, hold a small dowel or plastic rod against your ear (against the bone, not IN your ear), then touch the other end against each component to see which one is vibrating the loudest.