Sound test
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Press any direction on the D-pad, followed by A, B, X, or Y
at the Nickelodeon screen. The sound that is played will
vary, depending on which D-pad direction and button were
pressed.

Level select
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Go to the trash heap during the second part of level 1. Press
L, L + R, R(2), Up, Down, L + R, Select, Start. The sound of a
scream will confirm correct code entry.

Make Grumble blink
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Press L or R on the screen that appears before the start of a level.

Make Grumble open his eyes wide
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Hold R + Down when Grumble appears before a level begins or at the
options screen. After releasing the buttons, Grumble will appear to
be angry.