Presumably it is a HSM lens?
If so it should be almost silent.
You could hear the OS system working on my Sigma 150-500 and also on focusing, so unless it is really noisy nothing to worry about.
If it's just as the OS is kicking in then yes, there is a bit of noise.
Turn the OS off and use the lens (just to check), it should then be virtually silent.
Sigma can be a bit lousy with their OS. My old 120-400mm sounded like stuck gears.