Strange litter box behavior

Question
We have a female cat we found several years ago.  She is about 4 years old, spayed.  In the last 6 months she has taken to "scratching" in the litter box.  She is not just doing her thing, she is in there 10 to 15 minutes, scratching and spewing litter all over the place (we do have a covered box but it is very noisy, especially at 2am, 3am, 4am, etc.).  She has a clean bill of health from the vet, no UTI, etc.  I do not know how to break her of this habit without fear of her no longer using the litter box and instead using the carpet.  Any suggestions?
Thanks.

Answer
Sharon,

My 9 year old cat does the same thing and I have tried various tactics over the 9 years that I have had her to stop this behavior without success. I wish that I knew how to help you, I can empathise with how annoying it is to be woken in the middle of the night by a cat covering and recovering her business in the box. If you happen to find a good humane solution to this one I would love to hear it. I am only sorry that I couldn't offer any help with this one, other than a good strong "NO" in the middle of the annoying noise. If you have any further questions or concerns, please do not hesitate to contact me again. As I said, if you do find an answer to this behavior, please feel free to contact me and let me know what you find!