I'm sure your mini is actually becoming louder, but there are logical and acceptable reasons for why it's happening. Like you said, an increase in the ambient temperature would make the mini louder because the mini as a whole would be hotter. Also, a computer fan would get louder as time goes on, especially if it uses a bushing and not a bearing. Mainly because the bushing or bearing wears out, so there's more friction and more noise.
So... your mini should be getting louder, especially because of the hotter weather you're having. I don't think it's something to worry about though....