It's a reference to mizuhashi_parsee, a youkai noted for her power over jealousy. Thus, in Touhou fanart, "paru paru" is onomatopoeia for being jealous.
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Meanwhile, Sanae...Paru-paru-paru-paru-paruI forgot my slippers in the changing room downstairs!!Nnnghh...Nnh...RetryI'll have to come back all the way up that stairway again!!AAARRRGHDaaaaamn! I bet everyone is having fun right now!Sanae was forced to stay at home because her work wasn't done....was having fun in a different sense.