I have the same mower, and it, too, makes some nasty noises if I'm trying to mow tall damp grass with the side discharge closed. I would suggest that you remove the bag, and use a side discharge chute, instead....until you get that hayfield mowed to a more reasonable height. The Noise you are hearing is probably the belt tension pulley slapping around as the blade begins to bog down from the excessive collection of grass....which eventually builds up such that even the engine cannot handle the load.
