BMW Repair: air conditioning, licensed mechanic, car manufactuers

Question
my wife's 1999 528i will not cool the air at engine idle speed, even though i can see the compressor clutch is engaged and spinning.  while in Park, we can rev the engine and hold it around 2000 rpm and hear the compressor chirp (don't like the sound it gives off) to life and the air will cool.  at cruising speed the air conditioner operates fine.  it's just at stoplights that the air cooling stops.  my licensed-mechanic neighbor checked the coolant and thought the pressure on one side looked awfully high (he had the low-side, high-side gauges and all that).  he bled some coolant out of the system, and then it looked low.  so he added some coolant back to the pressure levels he thought seemed more reasonable.  the cooling behavior is unchanged - still get cooling at around 2000 rpm.  he said that chirping might be a sticking expansion valve or something, but admitted not being familiar with the bmw (he was a chrysler mechanic).  so what do you think?

Answer
The chirping is the high pressure switch popping.The system needs to be evacuated and recharged with the correct amount of freon.A/C is the same in all models,they may have different names for the parts but they all work the same.I have been to a/c classes in 4 diffeerent car manufactuers and they all work the same.