I've had an Antec 300 case for about 9 months now, and lately it seems like its making more noise. Can't be sure though as I have only my ears to rely on... Stopping each fan by hand, it definitely seems like Nexus Real Silent (which I use as intake) is making the most noise. The Antec fans which came with the case only make the typical noise of moving air. But the Nexus also generates an annoying buzz in addition to that.

http://www.youtube.com/watch?v=pfD7PKVfmLM

Is it typical for Nexus Real Silent fans to make a buzzing noise like this?

Maybe I'm nitpicking but I want to get a second intake fan and I'd rather it didn't buzz I love how cheap these Real Silent fans are, but I can also get some Scythe Slip Stream or Enermax TB Silence for just a little higher price.

Have you tested it without the intake restriction of the case, when one of my fans ramp up it makes noise but i know this is due to restriction on the intake.

If its silent when not attached to the case, try lowering the fan speed if you can get away with it, its either that or going to the
pains of making the intake less restrictive by cutting the metal grill out .

Yeah, I took it out and it's making the same noise. Just wondering if it wore out during the 9 months or that much noise is normal for these fans.

Does cutting out the metal grill have a real effect on noise or air flow?

Depends on the grill, if the noise characteristic doesn't change when the fan is running in clear air then cutting out the grill will likely have little effect.

Its not normal for Nexus or any good fan to wear out in 9 months, i have Nexus fans that are over 5 years old and still totally silent so it does seem odd that yours is like this.
The only other thing i can think of is lubricating the bearing,
If you carefully peel the sticker of the back of the fan you should be able to remove the dust cap/plug, add a few drops of oil into the bearing spin it by hand a little to work the oil in then replace the cap/plug and sticker.