So I have been hunting down a rattling noise underneath my Tahoe. It only makes rattling noise 10-20 mph. first mechanic said its the rear differential. I checked the slip yoke it was dry I put some substantial amount grease. i checked the u-joints looks good condition. minimal leak on the rear differential maybe a drop or 2 per week. I tapped off my rear differential make sure its not bone dry (200 ml or less). test drove for couple mins the rattling noise was gone. added couple more laps rattling noise came back
I used 75W90. I wonder if i need a thicker gear oil?

 

I would pull the plug on the rear diff and check it to see that there is fluid in there. I believe the correct level is almost up to the fill hole.

 

yes i added fluid until fill hole was dripping

 

Hey! That rattling noise in your Tahoe can be frustrating. Since you checked the yoke and u-joints, that's good. If you added grease but the noise returns, it might indicate that the differential needs a more thorough inspection. 75W90 is standard, but sometimes a thicker oil like 80W140 can help reduce noise.I’d also recommend checking for play in the differential itself. Worn bearings can cause similar sounds. It might be worth consulting a mechanic who has experience with these models. Good luck!