mouth, opening
谽 centers on a wide, open mouth or a gaping opening, like a valley or a mouth stretched wide.
谽 combines 谷 (valley) and 含 (contain, hold in mouth), suggesting a valley-like opening or a mouth that holds something wide. The exact historical development is uncertain.
A valley 谷 that contains 含 something inside: picture a wide-open mouth like a valley, ready to swallow.
For カン, imagine a wide-open mouth shaped like a can: can -> カン, and the valley-mouth opens wide to swallow it.
—