So we all work on servers, and some of us work around them as well.
The sound is annoying and bad for our hearing.
I wear earplugs when in the datacenter or earbuds which act kind of like earplugs with the bonus of music.
I've seen and played with noise canceling headphones but don't know if they allow you to play music though them, and also they're expensive.
Anyone have a free noise canceling program for Windows? Does one even exist?
How do you deal with the noise?
Wear hearing protection. Ear plugs are okay, the big ugly earmuffs have a better attenuation than plugs. Wear both if you need to.
If you don't you will ruin your hearing and it will never come back. Hearing aids are expensive...
If you ever find your ears ringing after doing something (working in the data center, hanging out at a bar friday night, riding your motorcycle, etc) that is your body's way of saying "your harming your hearing. If you don't stop you will lose it forever, and it may already be too late." Eventually the noise (data center, bar, motorcycle) won't bother you but you'll also hear a ringing / single pitched noise in your ears when it is quiet and when you try to talk to people with background noise, you won't understand a damned thing they say. All this happens because you've lost some of your ability to hear in certain frequency ranges...
I modify all my personal computers to make them quieter, but such things aren't really possible or appropriate for a data center. You certainly can't run a program that makes the computer quiet -- it is all those damned fans in the 1u racks that spin at 7000 rpm and they just make a lot of noise.
Noise canceling headphones work by generating an opposite sound of the one they cancel. The whine of servers is a perfect candidate for noise cancellation since the sound is constant, hence the sound wave is constant (not really 'constant', but it's shape stays the same). The headphones pick up on this, and generate a wave to interfere with that in destructive manner so reduce the wave to virtually nothing. They work awesome. Yes, the idea is you listen to something through them, they aren't meant to just be worn. I don't know what you'd mean by a program for Windows though...
In your situation, simple hearing protection like ear plugs or other would probably be appropriate, but if you want to listen to music, noise canceling headphones would be a must.
How I deal with extended periods in data centers is twofold:
Musicians earplugs, they're fitted, block 15dB across the board and are the only form of earplug I can stand.
Closed headphones, I have a pair of AKG 240 Studio headphones I carry that drop over 10dB themselves. Plus music or even phone calls when plugged in to my iPhone.
That combination is also what I wear on planes (that ~17 hour flight to the US is a killer, worse then the same time in a datacenter). The downside is that combo costs ~AU$500, but I think my hearing, long term, makes it worthwhile.
As chris said, if your ears feel at all fatigued, or ring, after working then unless you're pumping music too loud, it's certainly an OH&S issue.
I wear Remington M-28 folding earmuffs. They have an NRR rating of 28, meaning it reduces 28 db of noise. That's not too bad and their very comfortable. Often I will wear ear buds underneath to listen to music or podcasts and I don't have to turn up the volume in order to cancel out the noise.
The noise in our server room is maddening without taking some steps.
If you work for a large corporation, have the corporate Safety Officer come in and measure the noise. It may not be as damaging as you think- or it may be worse.