e*thirteen cassettes are designed for long term, quiet operation. However, like most high-precision bicycle components, periodic maintenance is required for best results.
Most cassette creaking noise can be resolved by disassembling the cassette, applying a liberal coat of grease to the contact points between the steel and aluminum pieces, and re-assembling.
Check out one of these articles to see how, depending on your cassette:
Special Note - Cassette End Bushing
All current e*thirteen cassettes use a teflon filled Delrin bushing to support the steel portion of the cassette. If this bushing is left out during assembly, your cassette will creak, and may be damaged!
It's a good idea to check for the presence of this bushing if you're getting persistant loud creaking from your cassette.