Motor Grinds with no power, but unplug it's fine

[andrew-caird 0]

Hi all,

My Ultimaker Original is working fine, but I just relubed the x and Y axis and noticed that it was grinding. I have modified my machine so that it has external motors and doesn't need the small belt. I eventually worked out that the motor was grinding, even with no power. Weirdly though, when I unplug the motor from the motherboard there is no grind at all. It's very quiet. Plug it back in, grind is back (all done whilst power off).

Very weird, any one come across this before?

Thanks,

Andrew - Australia