It sounds like your pressures are a little low. Often times when they start to drop there will be a buzzing or whistling sound behind the dash due to the a/c line that runs to the evaporator core inside the cabin. I would have the system evacuated to check the charge level and then have it leak checked. DO NOT just put in more refrigerant from the can you get at Wal-Mart or wherever, there is no way to know how much is going in that way and an over-charge will do some serious damage to the system.

__________________
Alex
MB Star certified, now independent.
Owner/Operator FDMotorsport Mpls. MN
Mercedes and GM certified transmission rebuilder
We all need a little help sometimes...

1998 E320 4MATIC-- stolen by my sister
1989 300TE-- sold
1999 C230 Kompressor