As well as being higher pitched, contra-rotating designs also produce more noise. The sound field is non-uniform (mostly axial), and there are claims of being

As well as being higher pitched, contra-rotating designs also produce more noise. The sound field is non-uniform (mostly axial), and there are claims of being able to reduce it through special phasing of the two rotors, using different numbers of blades on each, etc. But I wouldn't be particularly hopeful.