As i wrote in this article i had issues with an high pitch audio noise on the microphone line of my Aten CS1734b KVM switch.
After quick debugging it was clear that it was induced by the optical mouse attached to the KVM USB port. It was not the mouse itself (tried various) and it was not the specific USB port (same with the others) and it was not firmware (as it was still present after upgrading).
Using an externally powered USB switch is a good and cheap workaround, but it means additional cables and energy consumption and the switching is noticably delayed because of the extra USB negotiation hop.
I contacted the Aten User Support and they were very prompt and friendly. Unfortunately, my used devices were long out of warranty and any RMA would have to be paid by me and exceed the initial investment by far.
Luckily, my dad found a way to get rid of the noise pitch by installing a filter :)
His findings are described on his blog: http://heller.dyndns.lrz.de/~heller/KVM/KVM.html
Thanks to him, i can now enjoy the KVM switch again, hopefully for a very long time!