Looks like the cooling system does the job. Here's some reviews:
MacWorldQuote:
The new Mac Pro also contains only a single fan. Located just below the top of the cylinder, this fan sucks air up from vents at the bottom of the computer, and over the thermal core to cool it, and then pushes the now-warm air out the ring at the top of the enclosure. Instead of the wind-tunnel-like noise levels you’d sometimes hear with previous Mac Pro models, the 2013 Mac Pro’s fan is generally very quiet. In fact, in normal use, it’s nearly silent: If you put your ear next to the vent on the top, you hear only a quiet hum. (Apple told us the computer produces only about 12dB of noise at idle, and roughly 17dB under load.) In my quiet office, I couldn’t hear the Mac Pro’s fan over the noise of my 2010 iMac, even when live-rendering some effects on 4K video in Final Cut Pro X. That’s not to say you’ll never hear the fan, but you should expect a much quieter experience than with previous models.
The VergeQuote:
love how quiet the machine is. It's impossible to hear over an external hard drive or ambient air conditioning noise, even under heavy loads. The only sign you'll have that it's cooling itself is the gentle rush of warm air coming out the top of the unit, which makes a surprisingly wide and uniform column of wind.
EngadgetQuote:
The idea, of course, is for the blades to effectively cool the system, but also to make less noise in the process. According to Apple, the new Pro reaches 15 decibels while under load, versus 30dB on the last edition. And when the machine is idle, it simmers down to just 12dB -- very similar to the lower-powered Mac mini.
Quote:
Throughout, the Mac Pro gets a bit warm, but it's rarely hot, and it's always quiet. For lack of a better word, you'd have to provoke the machine to really be bothered by the heat: The warmest area is at the top of the chassis, and even then, you'd have to be sticking your hand near the vents to feel it. Otherwise, the chassis does get a tad warm -- and can take a while to cool down -- but it's much cooler than the air blowing out of the top. Avoid sticking your fist into the opening at the top and you'll be fine. As for noise, I tried hard to get the fans spinning, but they stayed quiet. Actually, if you put your ear up to the opening at the top, you will hear a faint purring, but again, you'd have to be the sort of wise guy willing to put your ear next to the hottest part of the machine (not recommended).