Annoying vibration...help!

[jekbrown]

Ok, my truck has an annoying vibration/noise its making and I thought I'd consult with some fellow 'wheelers to help me figure it out. Heres the info on my truck... '85 Jimmy, relatively new crate 350, TH700R4, rebuilt np208, 10 bolts f/r, warn full-floater, 4.56 gears, True-Tracs f/r, Rancho 4" lift (blocks in rear), ORD 1" body lift, 35s. Not sure what other details I could add...so I'll just move on to symptoms. Well, all is good when driving at low speed. But, once you get above, say, 30 or so...if you slowly let off the gas a relatively loud vibration noise starts....if I give it some more gas...it goes away...if I let off the gas completely...it goes away...if I just keep the ammount of pressure on the gas...the noise conitinues. At freeway speed, the noise is pretty loud. =( Steering etc isn't really effected when the vibration occurs...the truck does seem to decelerate *slightly* faster than it normally would with the same rate of decrease in gas. From inside the cab it sounds like its coming from the tranny area...but as many of you prolly know, hearing something from inside can be VERY location-deceiving. Its not anything in the rear end, already had that checked. Its not the drive shaft...its been lengthened/rebalanced etc. since the lift. I don't think its the x-case...its been rebuilt since the noise started...no change. I'm not an automotive guru...but I am pretty much at a loss. I have assumed for a while now that its a tranny problem, and am thinking about getting a Jet T700 to replace my 700r4...but if thats not the root of the prob, I don't wanna sink that cash into it right now. Any ideas at all? Could it be weak/toast engine or tranny mounts? Anyway, I'm living in fear of actually going 'wheeling until I get this problem solved, so ANY info would be greatly appreciated...if there is an data missing that might help with diagnosis, lemme know I'll pass along the scoop. Anyway, thanks much in advance!

J

 

[wayne]

I would say start with the mounts. Get a set of polyurethane mounts.

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<font color=blue>Happy Trails<font color=blue>
<font color=blue>Wayne<font color=blue>

 

[FRIZZLEFRY]

Have you checked your pinion angle?A friend of mine had this same problem and we chased it around for a couple of weeks and he ended up needing degree shims under his springs in the rear.Couldnt hear a thing on accell but over a certain speed on decel it sounded like something was gonna blow up.

All work and no beer makes Frizz a dull boy

 

[jekbrown]

I hear people mentioning pinion angle all the time...but being the clueless newbie I am I don't know exactly what it is...or how to measure it. Someone help the newbie!

J

 

[backlash]

When I put my foot on the brake and hit the gas, I feel something move. No, haven't really checked it out. But think it's wrap or bad tranny or engine mounts. When I get up to 65 mph I have a pretty bad vibration. Tires were just balanced, tranny's been out several times (different converters tried), everything is stock, seems to vibrate more under load than coast, but not always. Recommendations on tracking this down. I'm thinking drive shaft, but also have never replaced mounts.

Thanks

 

[rich'90]

did you take out your driveshaft when you did any of the mods? did you mark the position it came out of if you did? i had a vib after i did some tranny work because i forgot that the driveshaft is baklanced and did not put it in the same position it came out. just a thought.
Good luck

Go heavy or Go home