POP! BOOM! BANG!.... Violent noise and shake from somewhere in the drive line.

[mxhowes]

POP! BOOM! BANG!.... Violent noise and shake from somewhere in the drive line.
1998 A4 Avant 2.8 AHA 5 speed manual 252K miles.
Problem..... My son said there were noises coming from the rear-end. I drove it slowly from my house on a slight decline everything seems normal but a soon as I hit a slight incline it sounds and feels like the driveshaft is broken and is going to fall out.
Note: I have done a lot of CV joint replacements in my many years of owning Audi's and VW's. This does not seem like a CV joint problem.
What I have done:
1. Got all 4 wheels off the ground with jack stands on the frame.
2. Removed the heat shield and inspected the center drive shaft bearing. No looseness or grease leaking and the rubber around bearing is intact. While I was under there, checked the fluid level front and rear, neither was more that 1/4" low.
3.Spun all the wheels, cannot hear any clunking or feel any sticky spots. No loose wheel bearings or lug nuts.
4.Went around and blocked one front and one rear wheel and when I rotated the rear wheel the front would spin in the opposite direction. Did this in every possible combination that I could think of....no noises.
My son (who has taken over ownership of this car) said he first noticed a shaking from the rear end when coming to a stop. Turned around got some of the pop,boom,bang...then got on the freeway and everything was smooth till he hit the road into our house and as he described it " things started shaking and making loud 'thunks' it felt like I had a flat spot on one of my rear tires"

Any ideas out there. If you have BTDT please point me in some direction
I have owned and babied this car since 2007 and 120K. Spent a lot more time working on it than I should have but there is something about it that makes me want to keep it running...forever.

Thanks in advance,
Jim

[walky_talky20]

Did you look very closely at the small universal joint in the driveshaft itself? When they fail, they will tend to be fairly ok until heavy load is applied. You can't really manipulate the joint at all in-place, but you can rotate the driveshaft and look at it closely with a flashlight. If it's got rust coming out of it or visible parts missing, then you have a problem.

[mxhowes]

Did you look very closely at the small universal joint in the driveshaft itself? When they fail, they will tend to be fairly ok until heavy load is applied. You can't really manipulate the joint at all in-place, but you can rotate the driveshaft and look at it closely with a flashlight. If it's got rust coming out of it or visible parts missing, then you have a problem.

As I stated in the original post, I inspected the driveshaft center bearing and it looks to be in very good condition. The u-joint shows no signs wear, rust leakage or looseness.

[walky_talky20]

I think next step is to take the driveshaft out. My guess is one axis of the u-joint is tight. But it's not possible to feel this out while installed. It will do exactly as you describe. Cruise is smooth, but hard accel or going up a hill will cause a vibration that sounds like it's going to come right through the floor.

[mxhowes]

Solved!
The problem was with the Torsen (Center) differential. Low on fluid. I didn't think that adding fluid would solve the problem because it only took 300ml to be flowing out of the hole (the book says 1/4" low is fine.) I took it for a easy, slow test ride and no loud noises but a lot of shudder even driving in a straight line. The next day I decided to take it out of the garage, drove back forth about 50' 4 times and didn't notice any shudder. Took it out for a test drive and it was perfect. I even got rid of my very slight 35 mph drive line vibration that all of my previous Quattro's have had. If you have any drive line vibration I would first get a bottle of Gear oil (mine was 75-90 Synthetic ..98 A4) and fill it up. It was kind of a PIA but the results were well worth it.

[walky_talky20]

Wow. I've never heard of that. It's surprising that it would generate any amount of noise in a straight line, unless the tires are not all matched. With front and rear axles moving at the same speed, the center diff should not be doing much internal movement at all. Also surprising how little oil made the difference. In any case, glad you figured it out.

[mxhowes]

Walky_talky20 ....you were correct on the diagnosis. I added fluid to my torsen and it purred like a kitten for about a month and then boom, bang & pop I was right back to where I started. It's been a very busy year for me and I finally got around to pulling the driveshaft and sure enough one axis was frozen solid. I got the replacement parts and was started to put in the new u-joint and center support together when I noticed there might be a dust/oil seal missing that sits in the center support and seals the rear u-joint yoke shaft. I posted a question about this a month ago and no replies. Sounds like you know your driveline quite well.
If you got a minute search "mxhowes" or
"Driveshaft center bearing front seal 30x55x6mm?"
and shine some light on this subject for me
I have a good picture but could not get it to post.
Thanks in advance.