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?

What did the other 6 cylinders have for compression? Leak down test would be the best way to see where the loss is. Did this happen all of a sudden? Two valves are unlikely to burn at the same time. I suspect a blown gasket between the two low cylinders.

Sorry, forgot to mention psi on the other cylinders, they were 150 after just a few seconds of cranking on each cylinder. One cylinder I cranked a little longer and it showed 160 or so before I let up.

I think it was gradual until it became obvious. Performance seemed to be tailing off which is why I did the plugs and wires change

Leak down test will tell you if its a valve or gasket between the cylinders. I would not run it. Blowing between the cylinders wears the block and head surfaces.

Put me down as another blown head gasket expectation. Not many other options with two cylinders side by side with low compression. Running it will kill the block and the head, so don't run it anymore.

At minimum, the head has to come off to fix it. Unfortunately, pulling that Hemi head would not be any fun with the motor in the car. There is not going to be a cheap, easy fix. Sorry.

Thanks, guys. I greatly appreciate your input on this.
Yes, it's not going to be fun to get just the head(s) out of there. The engine is pretty much shoe-horned in there anyway.
I think i've only put 20 - 30 miles on it since I became aware of the problem. We'll see

Do not rum the motor until you get it apart and fixed, let alone drive it

i'm in the head gasket camp as well.
as has been said, don't drive it. that will cause a groove to be formed on both the block and head surface, [if it hasn't happened already, driving it the 20 miles you stated] requiring the engine to come out to get the deck milled.
that may have ruined the block if the groove is way deep, or may cause you to use head gasket spacers if they are available for that engine.

Got the head off and.... yup, it's the head gasket. So ugly I had to take a picture of it. There is a section of gasket missing between cylinders 6 and 8 that is about 3/4 of an inch at its widest point. It appears that the head and the block made it thru without any damage. I'm going to have the head checked out by a shop and they can tell me if it's any good.

It's a Fel-Pro gasket (#7728) and I can't seem to find anything with that part number on the FEL - PRO website. Anyone know of another source?

Not sure if Hot Heads deals with the smaller Hemis, but they'd likely know a source.

Did you by chance re-torque the heads after break in? it's something i was taught to do many years ago but a practice that has gone by the wayside

Put a straight edge on the block also!

Here's your head gasket. One Felpro 7728S and one Victor 1109.

https://www.ebay.com/itm/295446382328

definitely put a straight edge on the block as well.

Now that the problem has been found, the big question is what caused the failure? This is probably something your not going to want to rep[eat very often (especially if it only lasts 30 something miles). Something caused this issue, I wouldn't call it fixed until you can identify the original cause, and make sure that was addressed.

It would be pretty unusual to have a blown head gasket without having a damaged head, or a damaged block, or both being damaged. Sorry.

You should use the thinnest feeler gauge you have, I use a .0015 or .0020, to see if the block and head is flat or not between every cylinder wall on the thinnest part as well as the rest of the block and on the head also, especially where the gasket blew out
Hopefully they are not hurt

Shine a flashlight there, you'll see any gap

I did not re-torque the heads after startup. There was a couple of years between engine build and starting it, and in that time I don't recall that ever being mentioned. Still, that's on me

My friend that owns the performance engine shop ran a straight edge over the block, and using a feeler gauge, he could not get the .0020 between the block and the edge, but with the .0015 he could make some progress (with a little effort) but not much. He felt the block was okay.

I believe the root cause of all this was the head. At some point in the engine's history, there was a shallow crack in the cylinder head in the area between #6 and #8. It was repaired, my friend thought they used 'pins' to fill the slight gap. It ran nearly the whole width of the head surface between the two cylinders. The head was then resurfaced to remove any excess metal in the repaired area. But they didn't get it all or as good as they could have. Running a finger over the area directly between the two cylinders you ciould still feel very slight unevenness in the repaired area. You guys know the rest.

Not sure if this was a contributing factor, but some of the cooling passage were blocked in the area of #8. This head buildup may have added more stress on the head and the gasket. I didn't drive the car that much so I never had the cooling system flushed in the approximately 15K miles the engine had on it.

I have some spare engines, heads, etc. to hopefully get me out of this. Some are 241, some are 271. I understand the heads of the 271 will fit on the 241 block. Will the larger capacity of the 271 heads be worth it?

I want to thank all who replied. I appreciate your input.

thanks for the update and the only thing I can add it that maybe the pins moved a bit during heat cycling if it was a soft head gasket vs steel shim that could have contributed as well