Even if you're hesitant to open up your Mac, it's time to take the intrepid route, put on an antistatic wrist strap (Rosewill sells a quality unit for $5 on Newegg.com), open your Mac desktop or laptop, look around, and blow out any accumulated dust with a can of compressed air or remove the dust for good with a small vacuum cleaner. This DIY cleaning is overdue at this point, it's way too cathartic for its own good, and the moment you turn your Mac on and hear the fan running smoothly, you'll be glad you made time to do this.
Replace the fan
You turn on your Mac, and the fan sounds like it's struggling, grinding, or not running at all. Time for a new fan.
The good news is that fans tend to be cheap and can be ordered from sites like iFixit or Other World Computing. Fans aren't that difficult to replace--for example, iFixit has a killer how-to video for the unibody MacBook Pro, and the site includes repair guides for just about every Mac you can think of, so it's easy to go at your own pace with the repair.
Yes, replacing a fan is a fairly major step, but it's also a pretty easy repair, so grab an antistatic wrist band, a plastic spudger, and the appropriate driver. It's time to replace that sucker!
The joys of replacing thermal paste
As beautifully constructed as your Mac tends to be, a point will come when the thermal paste--the paste that's used to fill the microscopic air gaps found in electronic equipment and that helps dissipate heat inside your Mac--will dry out and become less effective in its task.
This is also the time when you're going to have to dig into your Mac, pull out the parts that have thermal paste applied to them, remove the old thermal paste with rubbing alcohol and cotton swabs, let those parts dry, and then apply a new layer of thermal paste before piecing your Mac back together.
The good news? It's not that unpleasant, and it's actually sort of fun. The bad news? This job gets a little messy, and it's somewhat intimidating to take apart your Mac's heat sink.
For the brave of heart, iFixit currently asks $9 for a syringe of Arctic Silver thermal paste, and a YouTube tutorial is available online. If you're somewhat experienced in taking a MacBook apart and want to get to the thermal paste-focused bits, skip ahead to the 7:30 mark, which explains how to take out the heat sink, how to remove the old thermal paste, and how to put everything back together again. All this is narrated by a guy with a cool British accent, which will make almost anyone sound smarter.
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