Our 97 T 100 has developed a whistle (when the air conditioner is turned on) that is heard in the cab. Is this a serious $ problem ???
Does it do it when the fan is on, but not the A/C? Could it be a belt noise?
The whistle might be from a defective expansion valve for the A/C when the refrigerant is converted from a high pressure liquid to a low pressure gas in the evaporator.
Does the sound change with the ac fan speed? If so, it could be a leak in the weatherstrip somewhere.
have you noticed a change in the sound when you are setting inside and windows are up vs down . could it be a window or door gasket ? turn a/c to normal and back to max with windows up couple times ,see if noise changes , if it is cab noise