Could you post a link to the Kalman filter you are using so i can see how it works?
If you are getting excessive noise it may be better to try and eliminate the noise before it enters the ADC instead of trying to filter it afterwards.
Check the power supplies you are using for the Raspberry Pi and the sensor to see if they are noisy. You could try adding capacitors or LC filters to the power supply to smooth the output and reduce high frequency noise.
You could try adding an RC low pass filter between your sensor and the ADC to reduce noise levels. Analog has a good article on filter design for ADCs on their website that may help you with your problem.