Harmonic vibration/grinding noise on light thottle...?

[kjdavis]

So Im trying to diagnose a problem that just started happening on my 02 F350. When Im on the highway, between 55-70 mph and I let off the throttle and keep it on lightly, around 1200 RPM I have a wierd whistle/harmonic grinding noise....very hard to explain. First I thought it was my turbo, but have pretty much ruled that out. Im thinking drivetrain now...

Am I correct that in 2wd the front driveshaft does not spin? It engages in the t-case right? If thats the case, Im thinking carrier bearing or ujoints. Moreso the carrier bearing because it sounds more like a grinding...metal on metal noise.

I have about 3K on the new 8" superlift suspension kit. I have vibrations and need to work out the pinion angle...

Any other thoughts? Places i should look? Wish I had a better way to describe the sound...

Thanks!

 

[DaveBen]

I think you made a good call. Pinion angle.

Dave

 

[kjdavis]

In my experience Ive never had a sound like this from pinion angle. The pinion is almost dead on straight with the driveshaft, and I need to bring id down just a tad...or make the carrier bearing spacers shorter to get a bit more angle on the driveshaft. I've never had anything with a carrier bearing though, so Im wondering if these can make a noise like that, and it just doesnt make sense that its only at a certain speed. But...things are strange sometimes.

 

[DaveBen]

You can get harmonics between the U-joints that can give you some odd sounds. The idea of the pinion angle is to keep the pinion and the transmission output shaft in the same plane; that is pointing parallel to each other. Here is what I don't know, and that is the center bearing angles. I am not sure how you adjust that angle or to what you set it to. Sorry.

Dave