With a 5 year old mower the drive belt could still be the problem.. It is the one hidden up in the frame. Unless the tranny has leaked I am sure it has the correct amount of oil in it. the fill will be located at the top, most likely with a vent tube hooked to it. But without knowing how much oil is in the unit there is no way of knowing what the correct amount to add would be. Hydorgear uses a 20w-50 oil in their units.
Check to make sure the bypass lever is pushed all the way in on the back, as well as look up into the frame to check the condition and tightness of the drive belt. A 5 year old mower belt may have cracking, glazing, and pieces missing out of it.
If the tranny is making growling noises you may have other more severe issues with the tranny. The growl is different than the normal whine noise like when you push the go control very quickly. Your neighbor didn't happen to pull a lawn roller or something similar with it did he?