Model AE 104, with an engine rating of 1580 cc. I have owned it for 3 years now. It has clocked 320,000 km. Of late, some whining noise has benn coming from the rear, presumably the diff. I have checked the hub bearings but all seem well. The noise is still present even when it is gliding on free, on a slope. The diff is full of cc oil and I change the oil every now and then. What could be this annoying whining noise?

SOURCE: Rear diff whining noise

Give the old man a pat on the back! There is an oil plug on the rear differential. On a level surface, remove the plug; if oil starts to flow out, the fluid is full. If no oil flows, carefully put your finger in the hole and feel for the oil level. Your finger should go no lower than the first knuckle before you feel oil. If it goes further, you are low on oil and should either fill it up or better yet replace it. Rear axle oil doesn't get replaced nearly often enough in most vehicles and since your truck is 18 years old, would be a bad idea to have it changed. Best of luck.
Greg