My jeep's engine is knocking.
Unfortunately without more information it will be difficult to narrow down why you are experiencing a knocking sound in your vehicle. In my experience Envoys have had many different reasons that could duplicate an engine knock. The most common is oil starvation. If engine oil is unable to travel through the oil passages to lubricate (this may be due to lack of oil or oil pump failure), metal to metal contact will occur on moving parts. At any time you hear an engine knock coming from your car, turn the vehicle off immediately and check the engine oil. Engine oil is probably the most common reason for the engine knock, especially in higher mileage engines.
Older engines with higher mileage tend to burn and/or leak oil between oil changes. When this happens the crankshaft bearings are usually the first to make noise. When you reach this point, it may be time to rebuild the engine. Rebuilding an engine would be a worst case scenario. If you have sufficient oil on the dipstick and the condition of the oil is good (not black, or contaminated with debris) the problem may lie in the oil pump or elsewhere.
Other possibilities include having bad or noisy bearings on one of the pulleys that drive the accessories. These include the alternator and the A/C compressor. To verify exactly why you are hearing a knocking noise in your GMC, it is best to enlist the assistance of a certified mechanic to inspect and properly diagnose your vehicle so you can get back on the road safely.