I have 18 and gear switching took me quite some time to get used to. Double clutch does that loud slap... but it really smoothed after oil change.
Also, as I normally tow mower at about 5-7 mph, up and down the hill, I mostly stay in the 1st gear. It's pretty whiny.
To be honest, I am not quite sure what you describe, though it sounds like as if you believe that it does not want to shift up(collar won't engage). Unless you have simple grinding noise?