'03 F-150 XLT...turning in reverse, rumbling noise that sounds like rear tires rubbing on frame..tires are correct size...only makes noise in reverse, while turning left or right. Have never changed fluid, since owners manual says the fluid is synthetic and never needs changing. Anyone ever had those noises and how was it fixed?

 

If it's a limited slip rear, the fluid definitely needs to be changed periodically, 75w140 synthetic and a bottle of Ford friction modifier.

Look at the axle code on the door jamb sticker, it's a LS if it's a letter and number, it's an open rear if it's 2 numbers.

 

Thank you! Will check when wife gets back with her truck.

 

Would tag numbers on diff identify type of diff?

 

Yes, but it's a LOT easier to look at the door jamb sticker.

Mine is a H9, which is 3.55 LS. A 19 would be a 3.55 open.

Tag will have a "L" in the ratio block if it's LS. If it's a 3.55 it would be 3 55 or 3L55. If you have a 4.2 or 4.6, it's a 8.8" axle, if you have a 5.4 it's a 9.75" axle. The size is on the tag after the ratio - 88 or 975.

 

Door jamb sticker has "H9". Based on other info you provided, I have a 4.6 engine so it's a 8.8 diff. Will get the gear oil and modifier. Thank you again!

 

Fluid change worked. No more rumbling! Thank you for your help with this, glc