Welcome to 2CarPros. Rumbling is commonly a sign that something has gotten into the squirrel cage on the blower and throwing it out of balance, or more likely in your case is that the bearings in the motor are worn and letting the motor shaft orbit inside them instead of spinning on its central axis. Sort of like a spinning top as it slows down and starts to wobble. Speeding it up causes it to stop the orbiting and then it works correctly.
I have also seen a few of these that failed because water got into the blower but those usually squeal from the bearings or they slosh when turned on.
To remove the blower motor is very easy to do on your car. If you go under the dash on the passenger side you will find a panel that has a few screws and push pin fasteners. Remove it and directly above it is the blower motor. It has an electrical connector and three screws that hold it in place. Remove the connector and the screws and lower it out of the HVAC box.
If you find nothing obvious like fuzz/water/a mouse or? Then it is likely the bearings are the problem.
Monday, July 29th, 2019 AT 6:02 AM