I once had a subwoofer that had an internal hum. It was actually more of a very faint buzz, but audible when it was quiet. The manufacturer of the sub acknowledged that some of their transformers were noisier than others and they sent me some parts to install around the transformer that physically isolated it from the cabinet and the noise went away. If in your case the service involved "disassembled and reseated everything" then I suppose it's possible that what you are hearing is resonance from vibration. They would need to loosen and retighten the fasteners holding the transformer in place such that it doesn't resonate quite as loudly. I just thought I'd share my experience since the solution for my issue was a physical one rather than electrical.
