I can hear a nasty "pop" sound too on my Mac. It sounds more like a "plop" than "schk" too me.
klatu-verata-niktum describes the same thing here:
It could be an extension. The only extension I had enabled just now was Adguard. The sound doesn't appear when quiting and relaunching Opera. Only on launching Opera the first time after starting up my Mac.
It is not only this Mac, I heard it on a different Mac too. Not just this version of Opera. A different user has experienced this too, months ago.