Keyboard Click-Changing volume whilst typing. iPhone 4

Have recently had my phone replaced by apple due to a battery failure on my last device.
I have now noticed that when I am typing that my keyboard click will go very quite from very loud (ringer volume set to max) and sometimes it will not sound at all. Then halfway through typing the rest of my TXT, Email or Whatsapp it will start sounding again.
This is a very random annoying problem that I never noticed happening on any other device I have had from apple.
Any help would be appreciated.
Thanks.
Oh and a hard reset does not resolve the issue.
iPhone4 iOS 4