Mi sono imbattuto in un forum Apple quando cercavi su google il tuo problema in cui una persona aveva lo stesso problema:
Eric's right, these adapters do get a bit hot and can make some sound, yet that sounds much too hot for safety. An AppleCare man told me last month that they were concerned to "capture" adapters that were getting too hot, and promptly sent me a new one in a box in which I was to put the problem adapter and return to them (all paid by Apple of course.)
My concern then was mostly with the strange sounds it was making, seeming to exactly mirror the CPU whine and activity sounds from my Macbook. Since the new one also did this, when I mentioned this at an Apple Store I was visiting the Genius offered to give me another new replacement. This one seemed silent at first, though on closer inspection in the quiet of home I found the mirroring effect was still present in the third adaptor, though a bit softer. As it happened I've ended up with a replacement Macbook because of another issue, making it four adaptors in my experience. All of them have gotten warm, and a bit hot sometimes, and all have produced the same subtle sounds of the CPU activity. I believe these sound effects result somehow as a result of the magnetic power cord connection, which sits very near the CPU. Unfortunately both my Macbooks have had the CPU whine (a high-pitched buzz) and this then gets reproduced by the adaptors.
So, I'd also recommend you call Apple or take it to an Apple Store for a replacement adaptor, which should not get as hot as you describe, but may indeed make some strange-seeming sounds.
Spero che questo aiuti.