I just bought a 1980 280TE wagon 4sp manual last week, and noticed yesterday after driving on highway and shifting into 4th gear, after driving about 10 minutes, a really loud metal clack, clack clanking noise underneath develops. It almost sounds like something is loose and hitting the tire..or coming off the car. I looked underneath and everything seemed to be in place.
The noise continues for a bit when I get off the highway and seems to stop when I'm going slow speed and increase when I accelerate at high speed. Also, I noticed the same clanky clacky noise briefly on left turn. I'll also mention there is some play in steering at higher speeds.
After the car sits for a while, or I'm driving on city streets (usually not for long and at the most 3rd gear) the noise isn't there. It seems to appear after highway driving. I tested it before on the highway and this just started happening.
Noise sound familiar to anyone?
The car has 200,000 miles on it.
Thanks in advance!

Sounds like cv joints/half shaft problem in rear end. Park so there is no pressure on the drive train, in neutral where the car cannot roll and get back there and try to wiggle the half shaft up and down. If you can, its bad.

Tom W

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

There is a small possibility that the catalytic converter is breaking up (internally) age,miles. This is happening to my car a 1977 450sl. Metallic (tinny sound ). good luck Abe G.