I guess the noise is normal then
it is very strange though, I never had this noise on any other tablet (I had a Nexus 7 for a month and then it was not charging any more - it was exchanged for a Galaxy tab 2 but it was a display model so we asked for a refund and got a Kobo Arc, we had the noise + reboot issue so we returned it and got a Kindle Fire HD).
I will be returning it on Monday for a refund anyway, the noise is very annoying and I don't like the sound quality nor the screen (the sound of our Kindle Fire HD is just the best ever on a tablet, it's even better than my laptop). The screen is nice but I prefer the smooth feel of a glass instead of the "rough" feel of the Kobo. I don't know which tablet I will buy though, it is not easy to find the perfect one
Thanks everyone for your help and feedback !!!