Try wiping all the under edges of the inside tank rim and lid edges and creases. When tearing down my last tank I was surprised there was a little gunk around in there and it was yucky brownish and a little smelly.
Little moist misdirected flakes or pellets of food and stuff is just a little smelly in general. So maybe that could help. I used a dry paper towel just so I could see what I had been missing!