I have been hearing extra noises when mowing and I trying to find?

I noticed 2 bolts rattling when engine in idle? I identified them as 2 bolts missing flange nuts # 14M7303 from the bottom of flywheel housing. I didn't how the nuts fell off, but I need to get replacements.

The next item is noises while the mower is running, I have greased the 3 spindles numerous times, but what about the gear case? How often do you add oil to gear case?

Anyone have other ideas?