N.B - THIS HAS HAPPENED OCCASIONALLY/RARELY
Heavy breathing sound (as in from extensive running) initializes after a very short spell - less than 30 meters moved since logging in. This may be temperature related - as HUD bars came up "cold" on joining server.
Also - seemed to intensify while crouched or prone. When in 'COLD' state, actively started up when dropping to crouch mode after few seconds (There were no breathing sounds before crouch)