I've turned it off in
Setting > Notifications > App Notifications > Whatsapp > Sound & vibration
Whatsapp > Settings > Notifications > Conversation Tones
And it still sounds. At one point I even tried to turn off all WhatsApp notif, but it STILL sounds even when no notification shown.
I just recently noticed this because usually I put my phone in Silent Mode. But now I'm waiting for a phone call, I can't do that, and suddenly noted how annoying this thing is.