How hot is your GPU getting? nVidia Specifies that the GTX 275 can run up to 105°C.
As long as you are not going over 90°C max loaded, you should have no issues running the fans at stock.
As for your more specific question, yes it is normal for the fan to make more noise.
When you turned up the fan speed, the fan started forcing more air through the cooler.
As more air is forced through it will get louder.
Think of it like riding in a car.
Putting around the city at 50kph you are not going to hear much wind noise.
As soon as you hop on the expressway and start driving faster, you will be able to hear the wind whipping around your car much louder.