It might be badly installed to the case, a loose screw could make it buzz. Maybe tightening everything up would fix it. Did you install it yourself?
Yes, but at the moment. it's not actually botled to the case, it's sitting outside it, so it's not that unless it's a component inside the psu itself. Also, the problem goes away if I take out the 9800GT - so it seems like it only happens when the PSU is running above a certain power draw.
If the wattage is sufficient don't be fooled in to thinking higher wattage equals more quality - you should get the lowest wattage that covers your computer, then find a good model. Once I bought an el cheapo PSU and it exploded after six months. 500W should be more than enough for most desktops if you actually calculate the peak power consumption.
I read on a lot of websites that Seasonic makes quality stuff, so that shouldn't be it - it might just be the one I've got.
How many 12v rails does the PSU have?
I have no idea. I can't find it anywhere, and as much as I've tried to understand this whole amps/rails thing, it doesn't make much sense, because it isn't clearly labeled anywhere. Is this it?
I read something about you're supposed to add the amps, so that should be enough, right? The card wants 26amps/400watts. I'm wondering since taking out the 9800GT stops the buzzing if I should get like a 700w one.
Thanks for the help, everyone!
Edit after above edit: Yeah, returning it sounds like a good idea. If the replacement still buzzes I'll opt for a higher wattage.