If your phone is making a strange sound right now then that is perfectly normal – unless it sounds like Coldplay. In which case seek help.
The UK emergency alert system is being tested today at 3pm. Phones, even if set to silent, will be sounding an alert. Partly this is to test how well the alert network is operating and to determine if there are any issues with particular phones.
The alert test is also an intelligence test which many people failed last time there was an alert test.