Thanks again for all the replies.
What I find so odd is how the buzzing intensity varies. For example, I can be reading an email in Thunderbird and theres pretty much no noise and then I'll select another email and the buzzing will start. Same goes for websites, spreadsheets, documents. In other words, the buzzing is intermittent and the noise level varies.
I listened quite carefully to a work colleagues dual 24" Dell monitor setup today and there was zero buzzing coming from either monitor. And that was with my ear on the top of the monitor! At home when the buzzing starts on certain screens I can hear it from the other side of the room!
Sigh.