does the shell around the fan/heatsink move around at all? I am not familiar with that specific model but I have had an issue with my Zotac GTS 250 where this would happen due to the plastic case around the heatsink/fan being loose. I would recommend starting there. I would first attempt replacing the thermal paste and pulling the heatsink whether its loose or not there may be a loose screw in there somewhere. Also assuming thats an older model you probably have had it for awhile, the paste usually needs replacing once a year or so anyways. Verify you dont require thermal pads before pulling heatsink cause if your card uses thermal pads on the memory its better to have extras just in case (most cards dont use these and most the time they are re-useable but I am not familiar with your model). So if the plastic case is a little loose but screwing down all the screws didnt help, You can do what I had to do. It sounds ghetto but I just took a peice of cardboard (Top of a cigarette pack) folded it a couple times and shoved it in between the plastic and the card, this applied pressure to it not allowing it to vibrate or move. If all else fails try putting your finger on it while its making that noise. you should be able to isolate where its vibrating, if its the fan itself you may just need a new fan.