I thought this article was rather timely today. Leo called our Heating and Cooling guy on Monday, and still hasn't heard back from him, and he lives down the street from us! Our AC has been making some really weird noises since last week, and we figured it was time for a tune-up anyway.
Leo went and got new filters yesterday, and when he went in to replace the current one, he noticed all kinds of build up of dust and nasty in the works of the system, so he just cleaned it all out himself. Took a vaccum, and went to town on it! And when I got home last night, it sounded much, much happier, and hell if I wasn't actually cold after a bit with the thermostat set at 74, even! So it seemed to have worked. I think he saved us a good $100 - $150!
Anyway, if you're having any issues with your AC making weird sounds, or not being able to cool your house efficiently during this random heat spike we're having right now, try changing your filter, and making sure your furnace is clear of dust and gook. It might save you a bit of cash in the long run!