Engine knock

[427TJ]

Stock '67 L35 396 (Camaro), over 100,000 miles, never rebuilt, leaky/seepy valve guides, fresh spark plugs just put in today, stock timing setting. Starts cold with no fuss, idles fine. (New Q-jet a year ago.) During idle warmup the engine has an intermittent knock. Not loud but it can be heard by a trained ear. I'm thinking it's from carbon buildup in the cylinders? Is there a fuel additive that will help get rid of any carbon buildup in the cylinders? The engine needs to come out and be rebuilt but I'm trying to put that off until I have the $$$.

Thanks in advance!!!