"My dear friend, now I have met My very old and dear friend Krishna on this field of Kuru-kshetra. I am the same Radharani, and now We are meeting together. It is very pleasant, but I would still like to go to the bank of the Yamuna beneath the trees of the forest there. I wish to hear the vibration of His sweet flute playing the fifth note within that forest of Vrindavan."
Sri Chaitanya Charitamrita, Antya Lila, chapter one, text 79.
A.C.Bhaktivedanta Swami Prabhupada, Founder-Acharya of ISKCON(International Society for Krishna Consciousness) writes in the purport that this is the verse composed by Srila Rupa Gosvami and it is included in his book Padyavali (383).
Now what exactly is this fifth note is beyond our comprehension since the flute vibration of Krishna is completely spiritual since Krishna and His flute are also spiritual.