What region are you located?
What FW version is currently loaded on the DGS?
While fans are generally source of noise, there can be some board level circuit parts that possibly over time start to degrade and fail or initially parts may have not been of good quality. I've seen noise in some older 100mb HP ProCurve switches. Soon as they are turned on, a buzzing noise is heard. And these don't have a fan.
I don't think there is any user replaceable parts with in aside the fan. Only thing you might do is contact D-Link and see if there is any option for RMA or some repair option they may have to see if the noise can be fixed if the noise is getting worse.