Chrysler sells a $8 spraycan of "Combustion Chamber Conditioner" for removing carbon deposits which has worked well for me.

This sprays a brown foam that contains both water and solvent detergents. You can sprat a bit into a glass jar and watch the foam separate into two liquids.

Last year i used a can of CCC specifically for cleaning out large visible chunks of carbon from the EGR internal passages inside the manifold and it worked faster than i expected.

The instructions call for spraying the CCC into the air intake while the engine is running, but many posters have removed the sparkplugs and sprayed individual cylinders. I did this one cylinder at a time and found that cylinders 7 and 8 on my engine seemed to be the ones that pinged.

If the knocking has continued constantly i would pull the valve covers and inspect rockers, springs, and valve stems.