Well, this problem that I thought was coming from the Radeon 4870 fan was really coming from the lower intake fan on my Mac Pro. The rattling noise was coming from this fan itself, and the Radeon 4870 was actually running siltent the whole time. This is probably why people with the X1900 and older Mac Pros have been reporting the same issue. The fix...this has actually solved the problem for me, my Mac Pro is running siltent now: I downloaded SMC Fan Control, then I increased the speed on my INTAKE fan to max, 2800rpm. I let the fan run at max for one minute, I had an idea that this would cause the fan to work out the kinks if it had any. Then I returned the fan speed to normal, and the problem has been gone for the past 15 minutes. I hope this was the permenant fix to the problem, and I hope this bit of insight will help somebody else with this same fan rattle problem. The problem is quite annoying, yet seems to be a common problem on the Mac Pro. You can download SMC Fan Control 2.2.2 (Snow Leopard compatible) here: http://www.conscius.de/smcfancontrol_2_2_2.zip Hope that this can help somebody!!