10%: CPU / case fans are louder. It adds more background noise but you can easily block it out because it's "low-pitch" so it doesn't stand out. I have a pretty big fan on the top of my case that's several times louder (to my ears) than the HD7970 fan at 10%.
20%: Slightly less loud than Intel's stock CPU fan
50%: VERY noticeable and MUCH louder than any fans in my case or my CPU fan (higher pitched)
75%: Sounds like a hair dryer
100%: Sounds like an aircraft jet engine
However, when allowing Catalyst to automatically change fan speed depending on load, I wasn't able to get it to automatically increase fan speed beyond 28%. 28% was when I was rendering Unigine with 16x AA and AF and tessellation. Get about the same result on Windows. So unless you're running a synthetic OpenCL torture test benchmark, you probably won't be able to get the GPU fan to go much higher than 30-40%. My operating environment is 70 Fahrenheit and I have a very well cooled case (many large fans), but I only use traditional air cooling, not water cooling.
Wearing my Sennheiser studio headphones that are "Passive Noise Cancelling" (they surround your ears physically so that air carrying sound waves from the environment can't reach your ears as easily), I can't hear any of my fans at all with the HD7970 fan at 25% or lower. I can barely make it out at like 30%. I can really hear it at 50%. By 75% or higher, it's completely obvious. At 100%, I can play loud music in my headphones and STILL hear the fan in the background as a constant source of noise no matter what I do.
P.S. -- I wouldn't advise running the fan at 100%. Make sure you operate your computer in a cool environment (AC or live in a cold climate), and make sure you have lots of low-RPM, large case fans that move a lot of air, and you won't normally need to run the HD7970 fan above 20%.