Plug buzzing - why and how do i fix it?

[Conrad11]

Hi there,

I recently bought a pair of Sennheiser RS120 wireless headphones and the work fine.

But when the transmitter base is not transmitting (automatically switches off when there is no signal) ...the power plug adapter that powers the base transmitter (bit that plugs into the mains) starts buzzing?

Why does it do this and how do I fix it?

Thanks.