My MacBook Pro 17", which I purchased a couple of weeks ago made some rattling noise from the area where the left fan is located. The rattling was not too loud but disturbing and noticeable from distances as 5 meters. It occured at any fan speed (which I tested with SMC fan control) but was most annoying at low fan speeds when the notebook was supposed to be quiet. I had no time to turn it in and had some difficulties ordering a spare fan from ifixit.com or other Mac spare part dealers. So I opened the MBP and had a look at the fan myself. I compared the left (noisy) fan with the right (quiet) one and did not spot any visual differences. None of the fans was clogged up or mechanically damaged. So I disassembled the noisy fan. Again I could not spot anything that could cause the rattling. However, when driving the impeller with my finger I could feel some increased resistance at a certain angle of impeller rotation. That probably was the source of the fan rattling. Now I removed the impeller (including the shaft) from the motor. Now I looked for anything I could mechanically adjust. As the fan is very simplistic, there is not too much. The only thing I spotted was the fitting of the impeller shaft into the plastic impeller, which is a tightfit design. ---- So I tried axially dislocating the impeller along the shaft by approximately 0.2mm. That worked. ---- I reasembled the fan and tested driving the impeller with my finger again. Constant resistance/friction at any angle of rotation (as far as that can accurately measured by my finger...). My MBP runs absolutely quiet since then. Btw.: I know, I shouldn't do that at home, while my MBP is still under warranty and of course any slipping off with a screwdriver could result in a few thousend $ damage. But my time is limited; taking it in for a repair had easily consumed a couple of hours. Doing it at home cost me roughly 15 minutes (including protecting the MBP mainboard with electrical tape before starting surgery on the internal organs of my machine). Conclusion of this post: axially dislocating the impeller along its shaft can eliminate annoying fan rattling issues. Hope, this post helps anybody, who needs a quick fix for a rattling fan.