* Improperly installed or damaged gears: This is the most likely culprit. During the rebuild, something might have been assembled incorrectly, a gear might be damaged, or a shim might be missing or incorrectly placed, leading to improper meshing of the gears. This is the most serious possibility and requires professional attention.
* Worn or damaged bearings: Bearings in the differential support the gears and their shafts. Worn or damaged bearings will create grinding noises that increase with speed.
* Low differential fluid: Lack of lubricant will cause excessive wear and grinding. Check the fluid level immediately. If it's low, it's a simple fix (but doesn't explain the *cause* of the low fluid). However, low fluid is often a *symptom* of a leak or a more serious internal problem.
* Damaged pinion seal or axle seals: Leaks would lead to low fluid, causing further damage.
What you should NOT do:
* Continue driving the van: Driving with a grinding rear end will cause further damage and potentially catastrophic failure.
What you SHOULD do:
1. Immediately stop driving the van. Further damage is likely to occur if you continue to drive it.
2. Check the differential fluid level: This is the easiest check to do. If it's low, you may need to have it topped off (and the leak source investigated). However, low fluid likely isn't the *cause* of the grinding, but rather a consequence.
3. Take it back to the shop that did the rebuild: Since the grinding started after the rebuild, they are responsible for rectifying the issue. This is crucial for warranty purposes. Explain the problem clearly and insist on a thorough diagnosis and repair.
If you can't get back to the original shop, you'll need to take it to a reputable transmission shop or mechanic specializing in rear differentials. They can properly diagnose the problem and provide an accurate repair estimate. Be prepared for potentially significant repair costs if the gears or bearings are damaged.
