As of the last patch, Nunu's ultimate started to give away his location and send players into a frenzy! They feared gone would be the days of sneaky Nunu ults that caught teams by surprise. Luckly for them, this has been confirmed as a bug and is on Riot's radar.
official forums.Confirmed to be a bug.
We are discussing solution/rollout options.
No guarantees when, but it will be fixed.