Because I've smeared it in shit.
No, seriously guys anyone know what's causing this? One of you more practical dudes must recognise the reason.
I'm connecting my phone to my amp via a 3.5mm to 3.5mm jack cable and whenver the amp is on with the cable connected the speakers hum until I play something. There's no hum during play but as soon as the track stops the hum will start again, unless I've queued something for immediate play.
If I remove the cable from the amp there's no hum, so it's clearly the cable connection.
In case you care or it matters here's the set up:
which I've connected to these
playing sounds from one of these