heavy breathing 
noun [mass noun]
Breathing that is audible through being deep or laboured, especially in sleep or as a result of exertion: the only sounds in the room were his heavy breathing and the loud ticking of the clock...
- Divine dreams of his precious student soon brought a soft smile to his face, his heavy breathing evidence of a deep sleep so rarely experienced by the opera ghost.
- He could hear breathing, heavy breathing that was not his own.
- The velvet darkness was still except for the regular, deep sound of heavy breathing, punctuated occasionally by a rumbling, contented purring snore.