My correspondence with Chord customer services is set out below. The short version is that the hissing noise is normal when using lower powered chargers. I’ve just continued to use the same charger and if the hissing is annoying me when using the unit I just unplug and recharge when I’m done...

Thanks mosterzero - very useful link with lots of info.

Long time reader, first time poster here! So I bought a Chord Mojo of Amazon (UK) last week, and I'm really impressed with the way it's driving my HD650s. The only issue I have is that just before it's finished charging it starts making a hissing/high pitched buzzing noise. I've read that...