Sounds like blower motor module resistor
it needs a blower motor control module located under the glove box
Have your blower motor resistor or blower motor checked out
faulty blower module. Under hood not far from blower motor
Your blower motor working even after the car is off is a good sign. This means that the car's system is preventing your car from overheating.
Ifit has the c68 option (auto air) check the blower control module under the hood
Are you getting this whistle when the car is stopped? If so, shut the radio off and see if it still exist. If it does, then shut the blower/fan down. If it goes away, then it is more than likely the bearings on the blower motor.
Look at the passenger side under the hood. The circular tube like thing is the blower. Unplug the wire from the blower to stop the motor. This will stop the motor, however, will not solve the problem. You may have to check the motor relays.
The switch in the dash may have faulted out in the "on" position.
The blower motor may shut off after only a few seconds due to a malfunction in the motor itself, a problem with the electrical connections, or an issue with the control system. It is important to have the motor inspected by a professional to determine the exact cause of the problem.
when my car did this it was a blown fuse
When the blower motor stays on it means there is a short to power, or the relay is sticking. 99.9% of the time the relay sticks. I belive on the 2004 silverados the blower motor relay is internal to the blower motor resistor. Try to find an auxillary blower motor relay and if there isn't one then replace the blower motor resistor.
BAD FUSE SOUNDS NUTS BUT IT IS?