Well, it isn't a transformer since the camera does not have one. The buzzing is probably something -like the shutter charge servo- not liking the low voltage it sees as a result of not having an internal battery.
Usually the way it works is the external source delivers enough current to charge the internal battery when the unit is otherwise not doing much. When it comes time to operate, the extra current comes out of the internal battery. Running only the external power means that the voltage will drop without the internal battery in there....
As I recall, these things used a 7.2v pack made up out of one of the variants of an 'A' sized cell. Your best bet is to hook an external pack made up of whatever cells you like, but attach it to where the internal pack attached and not try and use the charge-in connector.
I had a 460c way back when, and I re-celled it, but can't recall exactly what cell they used now. I think it was 5/4 AA. Plus, it may be that the NC-2000e was different, but I think all the 400 series (which the 2000e is based on) used the same packs.
I know what you're thinking. 5/4 AA? what in heck is that? Well, that would be the same diameter as a bog-stock AA cell, but somewhat longer.
Here is a link to confuse you even more.http://www.powerstream.com/Size.htm
I stuck the link into a new thread in the generic subject forum for future reference for battery cell sizes as well.