The hard disk spins constantly, as it is always recording from all three tuners. And that's fine: it's best not to routinely unplug it, partly because it needs to be on to record things and to download the EPG and updates, and partly because switching on and off, and spinning up and down, stresses the hardware a bit. It's designed to be on all the time and is happiest like that.
Why is it annoying? It's very quiet, and only uses about 20 watts of power.
It's only television.