Recently my Nokia 5800 emits a series of very loud beeps when I was talking on the phone. A quick check on the internet for answers to this problem revealed that the profile setting in my phone, more specifically the Warning tones, need to be changed.
So on my phone, I went to Settings > Personal > Profiles > General > Personalise. Then I set the Warning tones from 'On' to 'Off'.
I made a call after that to my family just to test and sure enough the loud beeps have gone.
Problem solved. Boy, was I relieved. Thought it might have been some bigger problem like some Nokia 5800 bugs that I heard so much about and which may be more troublesome to fix.