jayern's

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?