Loud Thumping '82 300TD
 

I just bought a 1982 300TD, the car runs great except for a few squeaks and thuds here and there. Basically I have no problems with the car when I am just driving around town. However, I have taken the car on the interstate between Chattanooga and Nashville (120 miles) 3 times so far and everytime without fail I get a loud thumping/banging noise that sounds like it is coming from the read end. The noise is so loud it sounds like someone is under the car banging with a hammer. The noise continues as I slow down but goes away when I stop. I have noticed it doesn't start until I have driven at least 60 miles. Also, when I hear the noise the car is still riding smoothly and there is no loss of power. Any help is greatly appreciated, thanks.

Muffler/exhaust hangers? I know mine are shot and I've been in the process of replacing them for 4 months now (read the lazy procrastinator hasn't started yet). It makes some noise, especially upon decelleration. I suspect as the exhuast system heats up the rubber rings may get enough stretch to cause the rattle and banging?

drive shaft flex disk?
any chance it is noisy only on turns? check your sway bar links. they tend to be quite noisy when they let go.

Does the sound come and go? Is it dependant on wheel speed? If so, I would check your axle half-shafts.

The noise is completely non-existent under everyday driving, it only occurs when I drive at freeway speeds for 50 miles or further. I have yet to hear it while driving around town. The trans shifts hard but it feels strong and I inspected the axel boots and found them to be old but not ripped, nor did I find excessive oil. I am stumped, I am about to bring the car to a garage, but I am afraid they will not be able to diagnose the problem since the only situations I hear the noise occur after driving for a long time...

Yes and yes! How does one check the axle half shaft?

Look for torn rubber boots. How DIY are you? The half shafts come out of the pumkin (rear diff), and go between that and the wheels. If they aren't torn, look for signes of fluid loss around the boots. On mine, there was a little metal cannister-type thing that had oil cakes on it. That means that the grease inside had been let out, and the CV joint went dry, which is no-good

The axle boots are dry-rotted/cracked looking, but I don't remember seeing any grease or oil, none of them were torn. I will double check tonight when i get home though.

Cracked looking or cracked open?
 

I brought mine to my mechanic twice and he could not find the problem even though I told him I suspected the half axles based on info I got from this forum and uploading an audio of the sound which only occurred after driving 10 or 15 miles. Sounded like dragging tin cans (ie cv joints of the half axle). I finally bought CVJ half axles from Denver Colorado since I had already eliminated the flex discs (Had them replaced) and found a local mechanic to install the half axles (was a family owned transmission place I found on the internet). He also drove the car and said "nothing wrong with your car"). I told him if you drive it 10 miles you will hear the noise like dragging tin cans. He put it on the lift and turned the axles and said "Nothing wrong". I pointed to the cracked boot and said "Maybe the grease leaked out". Nope. He went to the next cracked boot and stuck his finger in. It looked like it was only cracked but it was cracked open and no grease. And I had no grease on my driveway. See if the cracked boot is really open. You cannot tell just by looking. See if you can stick your finger in the cracked boot. It took me a year to find this problem since I had to drive 10 or more miles for the noise to occur. Since the axles have been replaced I'm driving all over the place with no problems.

My '84 300D had the exact same symptoms, and it was a bad half axle. In my case, the boot was torn, so it was easy to spot. Not much you can do other than replace it. If you're lucky, you might be able to find a good one at the junkyard. I did. Otherwise, you must choose between new and rebuilt. If you search the forum, you'll find lots of threads on here about the virtues of either.

Yikes, a new one is $371! How much life you possibly expect out of a rebuilt one, say from this site, I see they have them for sale here.

Is there an Advance Auto or Parts America in your area? If so : http://www.peachparts.com/shopforum/diesel-discussion/191042-advance-auto-you-area-new-axles-%3D-%2490.html

I had a similar problem on my 85 TD. It was dry half shafts and maladjusted SLS holding the shafts at an unusual angle. Check to make sure your SLS is holding the car level and then pack the half shafts boots with the correct grease. Solved it for me. (StarTech in Denver diagnosed it. I thought it was a wheel bearing).

Not from personal experience, but I've HEARD horror stories about Advance, they have a life time warranty, you'll be replacing it for a lifetime!