Please XOR your hashes instead of adding them! If you add them, you're losing bits on the low end. EDIT: No you're not. It feels like you should be, but with unsigned overflow, this actually works just fine.

This assumes of course that you're using proper hashes that make use of the full domain of the output type (a proper hash will have a 50% chance of any arbitrary bit being flipped by any change to the input). But if you're not using proper hashes, you're doing something wrong.