1980 280TE- Loud clanking noise underneath after driving high speed

[MissVV]

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!