Can a time capsule be set to sleep?
The Time Capsule is part wireless router and part hard drive. The wireless-part is always on, and creates a constant low humming from the fans, there isn't really anything you can do about that, other than unplugging it whenever it's not needed. If it's part of a larger WiFi-network and can be spared at some hours, that might be an option.

The hard drive's fans are a bit louder because it needs more cooling. The TC is meant to be silent when the disk isn't being accessed, and it shuts down the disk automatically after 5 minutes or so (not exact) of disk inactivity.

If your TC is constantly whirring like the disk is being used, make sure it isn't! If it isn't, you could try unplugging it from the power for a few seconds and turn it on again and see if that helps. If it doesn't help, there might be an issue with the disk and it's fans, and you might want to contact Apple for technical support.

I can't think of a way to manually putting it to sleep or power down the hard drives, other than disconnecting the disk from all your computers (AirPort Utility 5.6 has a nifty button for this under Disks).