Either Dell's quality control is lackluster or I have very bad luck. Of the four Dell monitors I've purchased in the past two years or so, two of them had the same problem:
After the monitor is turned on, an intermittent popping noise emanates from the plastic border, sort of like a clicking sound. It's most likely caused by expansion of parts due to heat. When I hold my finger on the case, I can actually feel the force of the pop. My U2713HM pops about once a minute after turning it on. After about an hour, it stops.
This same issue occurred with a U2410, which I returned. (I posted about the U2410 in these forums.)
I don't have the patience for exchanges and the panel lottery. I'm already on my second U2713HM. The first had two defective pixels.
This is a colossal waste of my time. It seems like Dell's MO is to knowingly ship less than perfect monitors, hoping that the owner won't bother to return it.
Note that I am neither nitpicking, nor am I hypersensitive. I'd be happy to email an audio recording of this to anyone who is interested.
It's pretty clear that Dell's monitor engineers don't account for thermal expansion in their designs. Why??? This issue has been going on for years, as evidenced in these forums. E.g.
http://en.community.dell.com/support-forums/peripherals/f/3529/p/18784679/18907665.aspx