you may have the minute alert enabled, there is a 'feature' that with cause the phone to beep at the end of each minute during a phone call so you can keep call length down, also the phone should beep when it is about to run out of batteries and may beep if you get a voice mail during a call

on the similar i330 the minute beep is in the phone preferences menu under sounds there is an option for minute beep with a check box next to it, this sound be similar to your version of the phone
tap the menu button when the phone dial pad is open to get to the phone preferences menu

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