He nodded thoughtfully and walked a bit more, in silence before he started speaking again, very carefully.
"The Master from my universe -- the completely insane one. I think he heard them nearly all the time. They're some kind of call or push to war. Except the war was the Time War and long gone by the time he got there and there was nothing for him to fight. So he found things. Made them, and not only wiped out most of earth, but was preparing to start an intergalactic war. My Doctor wanted to help him, but the Master shoved him away with both hands. Wouldn't let him and they just got worse. The one here I know best doesn't hear them anymore - because he accepted the Doctor's help in... psychic surgery, I guess, to get them out. They can be stopped, Jack."