I had a similar problem with my 9700 pro for the longest time, it would just randomly freeze in game / direct3d / whatever used 3d. And it would make a bad hissing noise.
I finally got around to calling ATi, and after about an hour of troubleshooting, they put me through to the head tech guys and basically there are a few things you can do.
If you have an INTEL processor, download this tool and install it.
http://downloadfinder.intel.com/scripts-df/Product_Filter.asp?ProductID=816
Also, go into your device manager and check if ANY items are using the same IRQ as your card. The ONLY thing that should share is a USB bus, anything else will likely be the cause of your problems.
If that doesnt fix your problem (this is really the continuation of the first step, but sometimes it helps)
go into your BIOS when your system boots up, and look for the following values:
AGP APETURE. It will normally be set at 128 or lower, if it's at 128, set it to 256 (should be the next available option).
in your POWER MENU, you should be able to find an AGP VOLTS section, set currently at 1.5v -- try 1.6, ATi does NOT reccomend 1.7, as it could cause damage.
Now, the next step they told me, I didnt have on my bios settings, but, they said if there is an option called SHADOWING, disable it, same with FAST WRITES.
But, you dont have to do that, as the card works flawlessly now. -- I've always found that any card problem is likely a POWER issue. So do the AGP Apeture and 1.6v change, and all should be good.