For a time now I had gave up on finding out what was the problem with a mysterious high-pitch constant noise in any sound recording in the macbook.
Today, I solved the mystery: It was my USB mouse.
So, here is some advice for anyone with weird noises on recordings: try unplugging everything you can unplug.
I don’t think this is an exclusive problem of the MacBook (3,1).
So I hope it is useful to someone else.
Also. Here is a funny mouse, working out. You are welcome.