Random 'bong' sound on 2026 iMac running OS Tahoe

Quit all running apps one at a time until you find the culprit. If you have multiple Safari tabs or windows open, close each one at a time.

Some "social media" / communication apps will make a noise when a message is received. Investigate them.

Check your sound settings, chances are that someone (or something like a cat) has modified a setting. You just have to do your due diligence to find what it is. It could be anything such as a notification that an e-mail has arrived to some notification from an app that you have installed, you just have to look in your system settings until you find the culprit.

Mac OS does not magically create noises so it has to be some modification that was done where the iMac lives meaning your home, office, etc....