I'm new here and just purchased a 2001 F150 SuperCrew A4wd, 5.4l, AT with a horrible vibration and deep humming noise. When accellerating from 0-30 everything is ok, from 30 to 70 there is a slight vibration and a deep humming noise that gets louder as I increase speed. It is annoying but if I take my foot completely off the gas pedal at any point between 30-70 mph, I get this really bad vibration. I can feel it in the seat, floor and a little in the wheel. When I purchased the truck a A4wd bypass switch had already been installed. I figured that out using this forum. The interesting thing is, when I flip the switch and bypass the A4wd I dont have any noise or vibration at any time while driving at any speed. From what I have read about the bypass switch, it only disengages the Transfer case. The drive shaft, differential, and axels still turn while driving. With that in mind the only thing I can think would be the cause of the vibration and deep hum would be the transfer case or a bearing of some sort. The upper control arms/ball joints, and inner and outer tie rods have been replaced on both sides, tires rotated, and wheels aligned. The clunking went away but vibration is still there. I'm confused at this point and don't want to start tearing everything apart without some insight/direction from others who may have had similar issues or the experience with this issue. Any assistance is appreciated!!!

 

If i understand correctly...this noise only happens when the 4 wheel drive is activated?
If that is the case...it would seem to point to the transfer case ( or T case). If you're lucky it is only low on oil....or you may have to over haul it

Other things also to check is the front drive shaft U joints and front wheel bearings

 

Thanks for the response. Thats what I was thinking. I checked the fluid level in the front differential already and that was good, I'll check the t-case next.