The Dlink DNS-320 suffer from very low transmission speed below 10 MB/s as well on 1Gbps network. The problem is in the firmware  version 2.03 is very slow when newest 2.05 has not problem to download about 40MB/s. The upgrade is almost obligatory for a normal use.

So reviews are reporting higher noise, it is caused by use of 1Gbps network speed then probably the temperature sensor is heated from the network chipset. If low noise is required and lower speed in not problem then fors the NAS used only 100Mbps network speed.