2010 Vibe problem

Not my own car: posting for someone else.
2010 Vibe was running very rough and vibrating. "Trusted" mechanic said it's running on only two cylinders, it's not an ignition or valve or
head problem, but something -- he can't figure out what without taking it apart, and he's never encountered this before on a Toyota engine -- "in the bottom end."
What "in the bottom end" could cause an engine to run on only two cylinders without creating loud noises?
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