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During gameplay, pause and press Up/Down/Left/Right/X/B/Y/A. The game will automatically unpause, and enable free movement by holding A and moving around with the D-Pad.
Contributed by MehDeletingLater
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Normally, the "Start Level" option in the Options menu allows only the first four levels to be selected. However, highlighting 'Start" on the title screen and pressing L/R/L/R/A/B/A/B/X/Y/X/Y will allow for all eight levels to be selected.
Contributed by MehDeletingLater
At the very end of the game's ROM is a secret message from the game's director, Axel Brown;

Dear Hacker, yes there are some cheats and other hidden stuff in Metal Morph.
Some are on the title page and some are not!! All standard cheats are possible,
you're just gonna have to find them, b-u-u-u-d-d-d-d-y-y-y-y-y!!!
---- axel@origin.ea.com


The spelling of "buddy" in this message may be hinting at a potential cheat code in the form of B, Up, Up, Up, Down, Down, Down, Down, Y, Y, Y, Y, Y, although the effects of the code and how to activate it (if it exists at all) have yet to be discovered.
Contributed by Dazz