What avast version do you have ?
The latest version is 9.0.2008, resolves some issues with the auto update notification popup.
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The popup is actually meant to be the auto update notification (check your avastUI > Settings > Update) and you will see that an auto update has just taken place. Or right click the avast tray icon and select about avast!. It appears that these tips/ad popups are piggybacked onto the auto update notification popup and this looks like it messes up the display and sound for the auto update notifications.
If you browse the forums you will see several topics relating to a known bug about updates.
1. no auto update notification popup (commonly displays an avast! tip - FB Privacy, instead).
2. no audio voice over for the auto update.
3. the popup delay timings don't appear to work either.
These tips popups have in the past been piggybacked with the auto update update notification, so the only way to get rid of them would be to disable the auto update notification, which if you want the update notification (when it works correctly) would remove everything.