I would say that you might want to tune your shrouds and stays, tighten up your spreader and then try a test sail. Spreaders should not point horizontal but have a bit of an upward tilt. Before you get underway tighten your shrouds just enough to take out the slack (both sides equally while the mast
is straight up and down). Real tuning is done while underway and requires a practiced eye. Shrouds are on the port and starboard side. Stays are fore and aft. If you've been down to investigate the keel then that shouldn't be it. The creaking might come from chainplates as they tighten and slacken and the mast
shifts a bit.
I know you know this but starboard tack is when your mainsail
and boom are carried on the port side and port tack is when your mainsail
is carried on your starboard side. I was a little confused by how you described which side it was that was creaking. I had an older Catalina
22 that would vibrate through the rudder
on one tack because the gudgeons were worn quite a bit. I know you said it was amidships that you had the problem so I wouldn't expect that to be of value.