Something the rather kawaii Pokemon
says constantly. The sound has the curious
property of driving insane most people above elementary school
after a few thousand repetitions.
An interesting factoid is that pika pika suru is a Japanese verb meaning to glitter, to sparkle. It's one of the many somewhat onomatopoeic words in Japanese formed by saying a two-sylabble sound twice. Another one is uki uki suru, which means to be excited. English has a few of these too: for example, the rare word "pell-mell" means in chaos, in disorder.