Ford Repair: p0171 lean code, mass air flow, vacuum leaks

Question
i have 2001 ford expedition 5.4 with p0171. there is no vaccum leak and the engine idles fine. also accelerates normal. the vehicle has 120k miles. the fuel trim at idle on bank 1 is 7.4%(long) -4% to 5% (short)/ bank2 is 2.4%(long) when i drive the fuel trim does not go over 10%.
mass air flow is not dirty either.
is there any common problems with this engine that set off an intermnant lean code for 1 bank?

Answer
the most common thing is vacuum leaks-if you haven't already i would smoke test it cold for a vacuum leak-maf dirty would cause and bank one and two lean code-you could have a weak or slow o2 sensor but i would wait until it wasn't intermittent to verify.