DEFAULT CLOCK...
When the timer starts, swipe down and tap on the timer so you can see it, then minimise the timer (swipe up gesture or home button). Alternatively, when the timer is running, on the clock screen as before, select any of the other options at the bottom (e.g. Stopwatch). Doing that should silence it but keep it running.
ALTERNATE CLOCK....
Install a different clock with timer. Open Google Assistant or say 'OK Google... set a timer' dont give a duration, just wait... Then when you're presented a screen to enter a duration, click on the clock in the bubble in order to change the default app to your 3rd party clock.
09-30-2019 12:30 PM