Vaccume leak?

Hi I have a 2001 f150 5.4 triton xlt and I had
a bad coolant leak for a while and it also
threw a bank 1 running too lean code. I knew
the leak was from the intake gasket so i just
bought a whole new intake and all the new
gaskets. When I finished I cleared the codes
and not long after the code came back up
again "Bank 1 too lean". I can definately hear
some sort of hissing under the hood but I
have not been able to find a vaccume leak
using carb cleaner. I replaced the pcv valve 2
times so i know its not that. I dont notice
any performance issues at all with the truck
either. It runs strong and idles fine. It dosnt
seem to get any hotter than usual either. I
cleaned the MAF sensor also. The truck has
240,000 miles on it. Im just really stumped
and cant think of what it could be. Maybe a
bad MAF or O2 sensor?