97 marquis/ which one is the charging----
 

My car is clunking intermittantly-which i think indicates the A/C comp is low now i want to put a can of freon in this car myself, which freon does it take, and the charging port -is that the one on the side of the dryer? the charging hose fits it. but theres a bigger port standing strait up what might that be for?? bob s
ps this clunk has been diagnosed. bob s