My 2005 Focus has reached 75K miles... I recently took it in for your standard oil change and the mechanic said that my air filter probably needed to be changed. Well, 3 hours later the mechanic came to me with a puzzled look. It seems that the Focus has a lifetime air filter and they didn't realize it. Guess I don't need a new air filter?! Since then, I've noticed a distinct rattling noise coming from the engine whenever I turn on the A/C. I took it back to the mechanic and explained that the noise wasn't there before they decided to hunt for the air filter. Well, to make a long story short, they had the car for almost the entire day, but can't find the source. The service manager states that it's the "lifters" on my engine making that noise. How can that be when it didn't make the noise before? The rattling is really starting to annoy me. Any suggestions?