Master System Converter/Power Base Converter buzzing noise

Just got a Master System Converter and when I hooked it up to my Mega Drive 1, a buzzing noise becomes very prominent in the audio.

This buzzing noise is not present when playing Mega Drive or Mega CD games, only Master System games through the Master System Converter.

Any clues about what might be causing it? My first thought is that it might be a power supply issue, as I'm using an aftermarket adapter (9 volts, 1000ma).

 

Mega Drive 1 is supposed to have 1200ma.

 

While a true 1A would certainly be enough for the converter (it's more or less a pass through device), I'd still try a different power adapter. Chances are that it's not actually capable of delivering the rated specs.