Does not do it everytime, but sometimes when going down the freeway and letting off the pedal and coating I get a weird knocking noise from under the car, can't really tell if its towards the front or the rear. I've been all over the underside and can't find anything loose, I've checked the following:
Entire K-member and front control arms
Rear lower CAs
Torque Arm front and rear mounts
Wheels
Motor mounts
Transmission mount
Transmission cross member
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It's an A4 car, the motor mounts are poly and the transmission is poly too.
Got a rubber LS1 mount ordered to get rid of a few other vibrations.
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Would a blown rear shock cause such a noise? I do get a slight vibration when its making the banging /knocking feeling on the bottom.
Going to remove the rear shock tomorrow and the rear end seems to bottom out on the exhaust very easily going over very small bumps in the road at about 25-30mph is why I'm starting to think it may be a shock.

It never does it when the TPS is above 0%....maybe driveshaft?? How tdo you test that to see if there's an extreme amoutn of play?

 

Well, after removing the shocks, there's nothing wrong there but I did notice alot of play in the drive shaft, I can probably turn it a little more than 1/8 of a turn in the opposite direction before it grabs the rear gears... so I think thats why it only does it coasting while the drive line is relaxed...
Maybe the rubber tranny mount will soften it up some.

 

i have an a4 and my car does this also

i do however have quit a bit of play in my rear end , prolly the clunk i get when coasting,

it could be rear end play , u-joints or suspension parts

 

Its the hamsters trying to get out

 

is a continuous knocking or does it happen just once?

if it's continuous, it's almost positively one of your universal joints in the driveshaft going out.

vibration is a very common symptom of this. carefully inspect the u-joints to see if the yokes are tight together, if there is any freeplay between the two ajoining yokes, then it's for sure the 'joints.

 

its continuous when it does it, and only on coasting. I'll inspect them this w/e, not sure what I'm looking for, but when you say "yolks" I'm assuming its the 4- yolks on the u-joint, not the tail shaft or differential yolk just before the u-joint.
I've never had a u-joint go out in my 20 years of driving so I've never had a reason to mess with one, the car does have over 90k miles on it....hell I had a Monte SS that I drove 233k miles and never had a problem..but not near as much hp
Thxs for the suggestion and help.

 

the yokes are the parts of your driveshaft that hold the ujoints in place. there is one at each end of the d/s, one at the back of the trans, and one at your rear end. make sense? ok just in case...

this is a universal joint:



and these are yokes:



what happens is the needle bearings run dry after so long and the clearance between the bearing cups and the ujoint center section become greater and you get a sloppy joint. this is why it vibrates. the driveshaft is actually moving around a little bit every time it rotates.

here's a cutaway of a ujoint:



if you look closely you can see the needles between the bearing cups and the ujoint center section. as that clearance opens up, those needles will actually fall out and then the vibration gets worse, quickly.

the rear universal joint went out in my fathers durango recently and its symptoms closely match yours.

 

ok, its this sentence that confused me, its not actually freeplay between the 2 yokes, but freeplay between each yoke and the driveshaft itself right?
The yoke in the transmission has freeplay in and out by design since it just slips into and over the tailshaft. All that would need to be replaced is the u-joints and not the 2 yokes, correct? I read your ealier explanation as if the yokes too would need to be replaced.
Thanks for the detailed views, excellent!

 

no, the yokes would not need to be replaced, just the universal joints.

 

I got under it and there's not any front and back play just pushing on the drive shaft. On the tranny tail shaft there is some up and down play just pushing straight up on the tailshaft yoke.
Would this still be a symptom of the u-joints or can you not tell just by simply pushing and pulling on the driveshaft? ...maybe I should make this another post on u-joint symptoms.

 

yeah none of the driveshaft parts should move at all (in relation to other driveshaft parts)

sounds like your front ujoint is worn. if you get under the car again look very closely at the bearing cups while you try to move the driveshaft around. what youll want to do is grab a yoke in your left hand, and the other yoke in your right. then try to turn the yokes in opposite directions, then switch and turn them in the other direction. look very closely at the bearing cups and see if there is any play in there. even a small amount of play will cause your vibration. you might want to take the d/s down and replace them anyway. it shouldn't cost you more than $40 for the new parts. the hardest part is putting the new ones in, those needle bearings are tricky.

 

There is absolutely no play in twisting motion at either end. The only movement at all is all in the tail shaft yoke of the tranny, it moves up and down enough where you can get it to make a knocking noise from within the transmission, independent of the u-joint, the entire yoke is moving up and down...maybe thats the source, I'll have to check my other cars and see if the tail shaft yokes do that.
All of the bad u-joints posts I have read have to do with a clunk when putting into gear or a knock ing noise only during accelleration...mines is only when coasting.

 

that doesnt matter, it could be either way.

try putting the car in gear with the parking brake off, try to rock the car back and forth to see if you can reproduce the sound, then try to follow it.

 

I've got the car up on a lift and can move the tires back and forth and it does make a knocking noise when changing directions with the wheels but its hard to say if its the gears in the differential or from the joints. Also hard to say its the same noise/feeling when driving down the road since there's so much other noise, its a vert.
Also with the rear end relaxed and not in the same position/loaded, it may be bonding the u-joints some so there's no play in them, I'll raise the rear up some and try it again.
I have some friends coming over tomorrow, I'll have one of them rotate the tires while I listen closer to each...or I guess I could drop the rear of the driveshaft so its only the rear end gears moving when I do that, put about a 20lb weight hanging from one of the bolt holes in the yoke to give it some resistance.

 

Midgets.

Damn midgets that get under cars with hammers.

You can buy some midget remover at your local hardware shop. I had to use some in my crawl space last fall, damn thing was infested with midgets. Took seven can of the stuff before they were gone, I almost called a professional midget exterminator at one point, they were nearly overpowering the spray.

They can really get into the tiniest of places.
I hear they can get up side the driveshaft tunnel and mess things up.

Get them out before they get too used to being in there. Make sure you use the spary cans, you don't want to get too close - they bite.