Chevrolet Repair: Tahoe AC Low Port Location, black caps, port location

Question
My 2003 Tahoe vibrates when I turn on the AC,  The vibration is only noticeable when I'm parked or at a stop light.  The AC blows cold air but I have noticed that it isn't cooling as quickly at it once did.  I was thinking it could be low on R134a.  Would I get the vibration if the system is low on R134a?  if so where is the low port located?

Answer
Vibrations from the A/C have been known to be the A/C line rubbing on the frame or body.  This line comes from the Accumulator(big silver thing located on the fire wall.)  it's the small line that goes to the condenser in front of the radiator on the passenger side.  The low side port you are talking about is located on this line.  You will see two ports, one small one big.  Most likely have the BLACK caps still on them.  The low side port is the smaller one.  Also, we have been having issues with the compressors themselves.  So don't just assume it's low.  Check the compressor.  Disconnect the belt and spin the CAGE....The front part pulley.  If it's tight, that's bad.  If it grinds, bad too.  You will need to turn it to feel how bad it might be.  But start with trying to move the line about a quarter inch from the frame rail first
Jimmy