Zennyo Ryūō is one of the Hachi-ryūō (8 Dragon Kings), also known as rain-god.
Kūkai also known as Kōbō-Daishi, propagated buddhism in japan between the 8th and 9th centuries, founder of the Shingon sect: esoteric buddhism, learnt after studying in china,
He is also known as creator of the terakoya (system of free education for everyone without disctinctions)
creator of the Jūjūshinron (treatise of the 10 stages of the development of mind),
popularized the Sokushinbutsu: entrance in buddhism mummificatition while alive by ascetism According to the Konjaku Monogatarishū wrote between 790-1180:
The emperor Saga requested to Kūkai by practicing the doctrine of the sutra of rain-praying (daiunkyō), to solve the long period of dryness which happens. After 7 days of ritual the rain-god appeared to him to let him know that everything will be solved, then a dark cloud arrived and rain was pourring down.
statue located to Ryusenji in dorogawa

PS: the sound is a practicing made everyday at the dragon dedicated