The latest update of the Google Assistant (“Ok Google”) has a very serious bug – when activating by saying “Ok Google” or pressing the microphone, you get a VERY loud tone, like a DTMF tone, which actually is rather dangerous if you are using Google Assistant to be hands-free, like riding a bike, motorbike or driving.
So far the only workaround is to roll back the latest update by choosing to uninstall Google Assistant, or the Google app, (which will uninstall updates) and then make sure to uncheck the Auto Update checkbox.
Settings > Apps > Google App > tap on the 3 vertical dots menu at the top right corner > Uninstall updates.
There are multiple threads on this, everyone is waiting for Google to do something about it:
Fingers crossed it gets resolved quickly!