When I shift my rear derailleur, there's a sharp creaking sound. Any thoughts?

I changed a flat at home, stuck the tire back on the bike, then went through the motions of checking my shifting. I can't seem to localize the problem, which are sharp creaking sounds every time I shift. It's not happening when I move the derailleur with my hands, only when I shift to bigger rings. Could it be within the component, or my dropout? I can't imagine that it's my frame since I rode the bike this week prior to the flat without any problem or messed up sounds.