Shoddy heat sink probably.. If you can, return it for some other brand.
XFX, Point of View and maybe Asus would ahve better sinks, or at least the Nvidia stock sink. Msi, Point of view and many others just slash some of their own cheap-o sink on with uneven bottoms and bad fans.
You don't want to replace the heatsink yourself unless you're V E R Y comfortable replacing heatsinks on exposed glass cores.
You'll slash your wrists and pull your hair if you happen to crack that bastard.