It's always DNS
Me to my boss the other day: "Hey, I was just in the server room and $OLDMACHINE is making a bad noise, I think it's on it's last legs. Is there anything still running on it?"
Him, half an hour later: "I checked and X, Y and Z are all moved off, but the only thing still in use is a couple of older machines using it for DNS".
It's always DNS.