My old Mac Book was getting hot and noisy but after 110 years I thought it was time for one of the new Mac Book pros. Migration assistant brought most things over. A few headaches with e absence of holes but ww won the day and got it working. Here’s the good bit - it started hissing intermittently. Hmmm, After turning lots of things off it was my ARC soft ware making the hius. Thankfully I had the authorisation codes - reloaded them and hey la! No Hiss I felt dead clver! I hope this may assist anyone else out there with a similar headache! Greetings form Perth Western Australia!