do you still have the plastic npi crossover or is it an aluminum one? if it's the plastic it coul've cracked and be leaking into one of the cylinders, could be intake manifold gasket, could be a head gasket. those would be the more common things.. then u get into serious stuff that doesn't happen alot like cracked heads and blocks
your intake looks powdercoated silver and they're pretty blurry so i can't tell?