P0171

Does anyone knows how can i solve a P0171 problem? The car also makes some noise located somewhere nearby the exausthion area (maybe the colector's area) when speeding up above 3.500/4.000rpm (but not in Neutral gear, just in move and often climbing! Confused

), it sounds like when you've got a big hole in the rear exhaust parts, do you know what i mean? Could it be from the catalytic system? Sometimes I've got also a DTC related with the lambda sonda, but i thing this could be just a consequence not the main trouble.
From a 2005 car with just 28.000 kilometers it seems a kind of strange, don't you agree?
Many thanks again!
Best regards,
Lambertino