Transformer hum is usually caused by core delamination. Unfortunately it will only get worse, if this is the problem. I'm getting a little hum on my 7075 and it sounds like transformer buzz to me. I haven't opened it up yet though to confirm. The only real solution is to replace the transform. Check for loose mechnicals first. The transformer looks expensive.

Note: This assumes the core is laminated iron rather than compressed ferrite. If it's a solid ferrite core then the problem is something else.