I have a MacBook Pro that I use for work when I’m on the road, and I recently upgraded it to MacOS 10.7 Lion. After a couple of days, I noticed that the fan noise was considerably louder than it was under Snow Leopard. I didn’t think much of it at first, partly because I had shifted back to a desktop Mac for other work I’m doing. However, I needed to switch back to the notebook, and the fan noise was driving me nuts. This is a machine with an SSD, so it is usually quiet (eerily quiet. I’ve never had a machine make essentially no noise at all). Lion changed that for the worse. Yes, I would get fan noise when I ran some intensive computing tasks. But when I’m editing documents, writing and testing code (either with Eclipse and Java for a data processing piece I’m working on, or XCode and Objective-C for some iOS projects I have), prior to Lion it was dead silent, and after Lion it was noticeably loud. Even with little or no CPU activity. It was rather annoying, particularly given the machine was nearly silent before.
After some digging, I discovered this Apple knowledge base article that explains how to reset the System Management Controller (SMC). I followed the directions for my machine, and so far, no more fan noise when I don’t expect it. It’s nice to have the silence back.
Update 2011-09-16: Fan noise is still back to normal (i.e. it’s gone), and I noticed that the machine isn’t running as hot as it was prior to the SMC reset. No perceived impact on performance, which is good.