drum carried on horseback
鼙 refers specifically to a small drum used by mounted soldiers, carried on horseback for signaling or military music.
鼙 combines 鼓 (drum) with 卑, which likely contributes the sound. The character was created to name a specific type of drum used on horseback.
The top part 鼓 is a drum, and the bottom part 卑 suggests something lowly or humble. Picture a humble little drum strapped to a horse, bouncing along as the rider signals commands.
For ヘイ, imagine a rider on horseback beating the drum and shouting 'Hey!' to signal the troops: Hey -> ヘイ.