It makes squealing sounds both under load and idle (There's no noticable fps loss when idle). Usually 2 or 4 cores are being used when playing a game. (Left 4 Dead 2).
Do you have the same general fps issue with other cards? Do you have another card on hand to rule out the GTS250 being the problem?
There may be an issue with the card the requires RMA due to the power stage (the part of the card making the odd noises that supplies power to the gpu core). I have fixed issues like this on my own so this one might just be a bad sample. If you have no warranty you can do a simple mod by applying a small heatsink to the mosfets and the choke coils (did wonders on my x1900xt and 8800gtx).