The '54 Dodge 241 Hemi that I have in my '36 Plymouth coupe started making an unsettling noise under acceleration, not a knocking or a sharp metal banging sound. More like a muffled, rhythmic contact sound you might hear if a tire had a decent sized lump on it. At idle and working the throttle by hand didn't produce the sound though the 'pleasing' lope to the cam wasn't there.
What I've done so far: Changed plugs and plug wires
Compression test. Cylinders 6 & 8 showed only 20 psi
Removed valve cover and observed proper action of the rocker arms and valve springs

I haven't removed the head yet so I haven't nailed down the culprit, but I suspect either burned valves (why two adjoining cylinders, 6 and 8?), or a blown head gasket between cylinders 6 and 8 (no water in oil or overheating).

I'm just a hobbyist, not a professional. What do you, more seasoned and experienced, people think?