1988 Chevy Cavalier Engine noise at highway speed

Just this past weekend, my Cavalier started making a very loud thumping noise when I was driving down the highway. The noise is coming from the engine, and starts after I have been on the highway for about 5 minutes. If I am driving on side streets at “city” speeds, it never makes this noise. But at highway speeds it does. When I pull off the highway, it continues to make this noise for a couple of minutes, and then goes away. I was told that it might be carbon build up, and put some of that cleaner stuff into my gas tank, drove around at city speeds for about an hour, and took it on the highway where it promptly started making the thumping sound.

I am not sure if I can call it a pinging sound... It sounds almost like a diesel engine...