1988 Cavalier; engine thump at highway speeds

Just this past weekend, my Cavalier started making a very loud thumping (maybe more of an intense wheezing sound) noise that is related to the RPMs of the engine 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?

Not enough information. Engine size and type? How many miles on this vehicle? Tell us as much as you can.

As McP said, lots more information would help greatly.

When you’re driving around town, do you accelerate hard? If not, try doing so and see if that sound occurs. Post back and let us know.

While ther are lots of possibilities, I’m wondering if what you’re hearing is the clatter of pinging. That sounds like a diesel and may be why someone suggested carbon buildup. Carbon buildup gets blamed for lots of things, including pinging, but in all honesty it’s rare that it’s the true cause. Carbon does retain heat, however, and it is an outside possibility if what you’re hearing is, in fact, pinging. A more likely cause is a malfunctioing EGR system.

Pinging comes from the cylinders reaching too high a temperature under load. The EGR system’s job is to displace a bit of oxygen in the intake with inert exhaust gas to keep the temp down. It’s like when you use a bellows in a woodstove and the fire gets hotter, then you reduce the oxygen input by closing an air intake and the fire gets cooler.

But based on the vague description it’s also possible that you have a simple misfiring cylinder due to a long overdue tuneup. When’s the last time maintenance was done?