I don't know if anyone could lead me in the right direction but I just picked up a used 03 F-150 Super Crew 4x4 (5.4L, lariat 43,000mi) and for some reason and I know noise resonates along the frame and the drive shaft's but it sounds as though there is a mild whining coming from the rear end at between 45-55 MPH and only at those speeds both accelerating and decelerating...It has the FX4 package with a limited slip axle just could there be something wrong with it or am I just not thinking correctly...Any help would be great....Thanks 30belowzero

 

Hi and welcome to the forum

Do you know someone with the same truck that you could compare with?
You could try rotating the tires in an X pattern and see if the noise changes at all.
Tire noise isn't too uncommon.
You could also stop by your local dealer and have a tech take the truck for a spin and see what he thinks. Usually a rear diff noise will show up more either accelerating or decelerating but isn't there while doing both. Notice that I said "usually"

 

Thanks for the ideas!!! I have tried the rotating already and that did not do anything(was worth the try). I will have to get it to a dealer to see what they might think it is...I don't think it is to serious since I had a 99' F-150 that had the rear axle bearings go on it and it developed more of a groul than the whine like have now....If anyone else has more thoughts pass them on...thanks

 

Do you know if the original gears are still in or has it been re-geared? Somtimes going to a lower gear ratio will cause a slight whine. 75w-90 synthetic is the new gear oil used on the 03's, and Ford dosn't recomend anything else due to bearing failure has occured due to thicker gear oil. That's what the tech said at the Ford dealership.

 

Actually I don't know if it has the original gearing, is there an easy way to check or do I just pull off the diff cover? I do know the sticker says it came with 3.55's limited slip...Thanks for the tip....

 

check your uneversal joint cause they can do the same thing