Play a 1080p youtube video for more than 10 minutes while on AC power volume muted and tell me you can't hear the fan.
You apparently have a magical SP3, either that or the i3 version has considerable less fan noise, which might be likely as it produces less heat.
I can confirm that, been looping a
2K 4K video for 15 minutes on my i3 and no noise yet. Silent, fanless, no fan, no noise, I even try to put my ear next to it. True for other cases as well, I only have the fan run when I set up the SP3 for the first time (updating, etc) or graphically intensive games like Asphalt 8 and desktop games.
EDIT
For those of you interested, I was looping
[Ultra HD 4K] STAR WARS 7 Official Trailer, downloaded with IDM in 4K quality (yup, 4K, not 2K not 1080p), and played with the latest version of XBOX Video app.
For 1080p, I have watched the entire "Earth to Echo" movie (the movie in the SP3 presentation when Panos was demonstrating the front facing speakers) without the fan running at all. Now go return your i5!
EDIT 2
I am now looping the same 4K video with both the XBOX Video app and PressPlay Video app side-by-side. Its been 20 minutes, and its still silent. It gets warm though. It boils down to either the i3 is impossibly efficient, or my SP3 actually has Core M in it.