hi all- new guy looking for a little support with my 2008 f150 supe-rcrew lariat.
I started on google which led me here- seems like a great site

here's what I need help with....

driving home from work- took a turn on a back road- heard sort of a "clunk" from the rear of the truck- the kind you feel through the floors. Immediately started hearing a grinding metal sound coming from the rear of the truck, but only when I accelerated- not while coasting. It sounded like what it might sound like if you drop a converter nut to the bottom of the bell housing and it just keeps getting chewed up-
Looked underneath- saw nothing. The sound was loud for about a day, and now it's hardly audible with the windows up, but it's still there when I accelerate / nothing when I coast- at any speed.
If I pop it in neutral and gas it- no noise... slap it back in drive and accelerate, noise is there. I'm also sure I feel a difference now when I'm driving under 35- I swear I can hear a slight "whirring" sound in the drive train and even possibly a slight vibration- very faint sound though. I never seemed to notice it before and now I do...

today- I supported the rear axle with stands and I crawled underneath- (yrs- front was also blocked ) with the tranny in "N" I spun the drive shaft with my hands- all sounded normal- no play in the shaft at either end, no visible leaks.
so then I started the truck and had my wife throttle the pedal in N and in D -
I could hear the "metal rattle"- it sounded like it was coming from the middle of the rear of the truck...

my thoughts are... spun a bearing, is it possible a bearing dropped somewhere in the housing and that was the metal grind I heard real loud the first day- and as the bearing or what ever it was continues to get chewed, the sound got more faint:

thoughts????

btw- it feels steady & smooth at high speeds- 65 - 80 mph. but when under acceleration or just when cruising, I can still hear the "sound"...

thanks all

 

also- the truck is approaching 68k miles

 

Make sure it is not a bad u-joint. With that amount of noise and grinding, it should be loose when you try to move the driveshaft (not turn, shake).

 

u-joints are tight- no play...
also- from "drive" to "reverse" and back again has no clunk, which should indicate a worn u-joint, I'd imagine.

thanks for the reply

 

Pull the inspection cover off. As the Differential wears the space between the gears increase until the teeth break off. Then jumpin and jeerkN happen with a lot of pieces happening.Along with the crackN and popN.

 

I'm thinking maybe a chipped or broken spider. Or if it's a trac-lok maybe the clutch packs.

Won't know unless you pull the cover as stated by Papa tiger.

 

Happened on a turn on a back road huh?
Check rear axle u-bolts and exhaust hangers. An exhaust pipe rubbing will transmit a huge array of noises.
Also, double check that rear U-joint and take a lok at the center carrier bearing if it has one.

 

I'll pull the cover off when I return after xmas-
I inspected the exhaust real good- that was one of my initial thoughts- I even stuck a huge neodymium magnet under the muffler to hold any broken pieces in place that might cause a rattle. I crawled back under today and looked at the ujoint again- it's as tight as new...

I'll post a pic of the rear diff once I remover the cover- I'll do it wednesday morning-

the part that has me stumped is that it only occurs during acceleration (forward or reverse) or while steadily cruising- but goes away as soon as I lift my foot of the pedal (at any speed).

I figure if it was something in the exhaust- it'll happen even in neutral- which it doesn't...

thanks guys

 

Look for a broken stainless piece above the exhaust parts also. Sometimes the noise transmits like at a different location.