I'd check grounds on the enginel, underneath on the frame, etc.

 

Knock sensors are cheap and the job would take about 2 hr to do. At the same time you are replacing the knock sensors, it would be a good idea to replace the intake manifold gaskets as well. And you can take the time to clean under the intake as well.

I'm actually about to take on this job myself on my truck. After doing a lot of work myslef, along with tons of videos on YTube, you be surprised how easy it is to do.

I got scammed once by a mechanic, which gave me a lot of motivation to do most of the work myself. The quotes I'm seeing lately on this forum for the various work makes me cringe. Also the money you save by doing your own work, can be used to get your tools for other jobs.

 

Just be sure you get actual ACDelco knock sensors.... the cheap ones are junk and its not something you want to have to do again. Also when the cheap ones fail, they are often so poorly made that when you try to remove them the outer metal shell which is what a socket fits on just spins. Then you have to try and use a large flathead screwdriver or chisel in the narrow hole to try and beat it with a hammer to get it to rotate. Been there, done that. Also go ahead and get the knock sensor harness too (Dorman ones are fine for that part) as the original rubber plugs which is the entire cause of the original sensors failing have hardened and let water get down into the hole where the sensors are.