Late to the party but thought I'd share my experience of the i3-6100 at stock clocks with the stock cooler as I've just popped one in a Silverstone Raven RVZ02 HTPC case.
The case is on the desk just behind the monitor, so quite close to your ears and easy to hear any sounds coming from it. The stock cooler isn't bad at all from a noise point of view considering its so close to you. On idle its not audible but you can hear it 'rev' up from 1000-1300rpm once the CPU starts having spikes in load. This is with the silent profile in the Asus bios. Under gaming load I'm seeing low to mid 70s and the noise is audible but still rather quiet. The case doesn't have any fans do just relying on the vents on the side for intake, then rear and bottom for exhausting the air blown off the cooler.
I'm going to try setting up a custom fan profile to stop the 'revving' and see how I get on from there, but overall the noise level is low