After mowing about 11/2 hrs. the mower lost speed on the left side drive. Also I could hear a chattering noise from the pump area. this noise got louder as time went on. I shut the mower down for about 30 mins. and restarted it and it had normal speed and was quiet. The fluid reservoir level is half way between the full and low level lines while cold. What would be causing the lack of speed and the chattering noise?