D-Link used a very very small fan within the router, to make it actually extract air the fan runs at high RPM. Don't think they would fixed this unless your router is overheating, the fan turns on depending on the internal components reach a certain temperature such as the CPU, or accessing HDD.
Though they could have a better range such as 50% - 100% fan power, to make it subtle. Is you fan on all the time?
It's normal for the fan to go on if you accessing the internal HDD, idling it's quiet.