Noisy enough to be audible in a quiet lab on my DS1074Z. Much noisier than my laptop or computer.
Does the internal temperature warrant this?
Hard to say. The fan is speed-controlled, likely from some internal temperature.
The usual issue with noise on these fans is that, just like on PCs:
* Fans don't do anything for specs, so they use fans that are reliable but cheap, so they the losing part of the trade-off is noise.
* The fan is smaller (cost), so much noisier (speed) than a larger fan with no easy option to replace it by something larger/slower.
In the case of the DS1000Z, the fan is 50mm and runs pretty fast. It's an uncommon size (for a PC fan), and with the high CFM spec, it's hard to find a good replacement that moves enough air and is quiet. I ended up picking up a Gelid Silent 5, which is substantially quieter, but still clearly audible. It's the only "quiet" 50mm fan I found that moves close to the right amount of air.
Laurent