I have taken a good long look at your laptop's inside (thank you so much for attaching photos!). The only moving part I could see is the fan. You have no hard disk and no DVD drive or anything else that moves, meaning the fan has to be the cause of the noise. It is an unusual noise for a fan, but that is the only possibility. If you tried cleaning it and that didn't work, then it probably needs to be replaced. There are no guides on iFixit for doing a fan replacement (or anything else about your laptop for that matter), but if you figured out how to get inside the laptop, then replacing the fan won't be too much of a challenge. If you get stuck, there's always YouTube! While you're at it, you could [guide|102668|create a guide|new_window=true] so someone else who is having a similar problem can have a guide to follow.
Good luck!