Hi @WokingBoy Go to Settings-Sounds and Vibration- Volume. Ensure the slides for Notifications and System Volume are set to an appropriate level. you may then need to check recent Notifications and check that the alert is checked for appropriate apps. An example for Gmail here:
Thank you but, unfortunately, the switch from sound to vibrate and back is continuous without anything else happening on my phone eg: no notifications, no messages, no calls. It just keeps going on and on and is driving me crazy.
@WokingBoy ok, thanks for confirming. you could try clearing the cache partition to see if that makes a difference :
Press the Power key to restart the device.
Factory Reset an option if all else fails though realise can be reluctance to do so,would need to ensure all Data is backed up obviously .