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The theme song to Psych, I Know, You Know, is performed by series creator Steve Franks and his band, the Friendly Indians. WIKI. You can download the MP3 from the link I am providing. These lyrics were all I could find off of someone's blog.
Ron Grainer wrote the tune itself, but it was an electronics whizz called Delia Derbyshire who turned it into the eerie sounds we now recognise as the theme tune.