Q: Jeep making a whirring noise

Hello, I will address the issues as you present them. For the whirring/humming, I recommend having this noise heard by a mechanic to have it diagnosed correctly. Noises are a hands on job, so they must be experienced firsthand to give a proper recommendation for repair. The loss of power and poor fuel economy can be related, because you had quite a bit of engine work done, there’s a chance it could be out of sync, have vacuum leak, or have a faulty sensor. This engine is also equipped with electronic fuel injection so it should be scanned for codes and data monitored to see if something is out of spec. For the U-joints and the rear driveshaft, and the belt, those won’t affect the drivability of the vehicle. You can take it back to the shop that performed the work or you can have another mechanic take a look at for an independent inspection.