About a year ago, North Porshe Dealership checked and recharged my A/C in conjuction with replacing belts, etc.
They claimed that the compressor had a slow leak but I declined to have the repair done.
At this time, it starts to make a whistling noise about every minute or two (which it did before the dealership charged it up)..
I went to an off side autoshop to have the A/C pressures which showed nominial values (within 5 psi) on the high pressure side.