Years ago, I had found it.
I was in an aquarium at the walrus enclosure. There were a couple of them swimming around, and this was the lower section, where we could watch them swim (instead of the upper level where we saw them lounge).
I walked up to the glass. One walrus came up to me. It pressed its face against the glass and started to whistle. I tilted my head and watched it. The whistle was clear and distinct, echoing through the room. Piiiiiiii.
I slowly moved to my left. It followed me. Piiiiiii...
I leaned lower to the ground. It followed me. Piiiiii...
I stood upright and moved to my right. It followed me. Piiiiii...
I love you, my dear friend, the whistling walrus! I hope you have a good life. I miss you and the one moment we shared.