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Allworth,
Turning "on" and "off" the A/C button engages and disengages the compressor. "On" puts additional load on the engine to run the A/C cooling system, which increases RPM at idle. It is normal for A/C "on" to cause a constant, noticeable but not overwhelmingly loud noise, especially under 2000 RPM.
Assuming your serpentine belts are in good shape and you have an appropriate amount of refrigerant in your system, there's not a cause for concern. It is almost a guarantee for an A/C system to get noisier as it ages.
Gregthegr8,
In general, the A/C system really doesn't need much maintenance. Belts are cheap and easy to DIY, replace them every 4 yrs/50k miles. If the A/C won't blow cold, look into getting a recharge. Otherwise unless something fails, there is no need to throw money at a slightly noisy but otherwise perfectly functional system.
Also I cannot stress this enough, do not DIY an A/C recharge! Yes, auto parts stores sell recharge kits. They usually forget to mention that these come with a significant risk of "overcharging" the system, which will destroy it in seconds and cost $1000s to repair. Just go to a local shop.
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