It could be ether. The only way to know is to change one at a time to see if the vibration goes away. You can ask the service company if they can bring both parts with them when they come to do the repair. Ask them ahead of time if they can bring both parts and change one at a time to see if it is one part or the other. They may charge you a restocking fee of one of the parts is not used and needs returning.
Depending on the cost of the parts. It may be best to change both as the tech will need to spend extra time trying each part to see which one it is. Labor will be a consideration for you.
Before the job is done, ask the contractor to supply you with the costs of the parts individually and the cost of the labor as a separate item. Also is there a restocking fee if parts are returned. I would think that it would be easier for the contractor to change both parts at the same time, so the amount of time may increase if they spend it trying each part to find out which one is the problem one. With the unit in the attic it may be harder to access.
By asking the right questions ahead of time, this will help you decide how to proceed.