This refers to the sound made by the camera constantly tweaking the aperture opening on some cameras while framing shots (or in video mode). For some reason this appears to be more of an issue with the small primes than the zoom lenses, and more with certain bodies (such as the GX1), but it is a fairly common problem. The problem seems to occur in very bright light when the aperture is at or near maximum opening. There are a few things you can do about it: (1) lock the exposure (and rattlesnaking usually stops); (2) on Panasonic cameras turn on Live View Boost (which lessens the problem); or (3) force the aperture to stay absolutely wide open (e.g., use an ND filter outdoors). None are really a "solution." Personally, I'm not very bothered by such noises while shooting stills, though if I were shooting video with these cameras, I'd have to do something to deal with such noises.