beeping noise when A/C is on at freeway speeds
 

Hello everybody i'm new and recently purchased a 1998 volvo v70 5cyl. non turbo. The wagon is in excellent shape with no mechanical issues except I hear this faint beeping noise coming from the dash area when the A/C is on. This noise only happens when i'm driving at freeway speeds, when I turn the A/C off the noise disappears......please help driving me crazy :)