Howdy! Long time lurker, first time post.

So, my Iron Duke started making a "ticking" sound that gradually got louder over time. Eventually got to a "slapping" sound. At first I thought "valve train". So, I replaced all the lifters and pushrods. It needed it anyway, as 6/8 lifters were in various stages of collapse.

Sound still there.

Then I noticed that it was more of a hiss/pulse sound than a tapping or slapping, and more on the exhaust side of the engine. Like the exhaust stroke was leaking somewhere. Cool, must be a bad exhaust gasket or manifold. Replaced both.

Sound still there.

Maybe it's a bad spark plug? Replaced all 4.

Sound still there.

So, I've replaced intake gasket, EGR gasket (and checked EGR valve, it's good), valve cover gasket (valves and springs looked good), exhaust gasket and spark plugs and exhaust manifold.

I'm now down to a possible bad head gasket or (I really hope not) cracked head/block. I also thought about the possibility of jumped timing gear, but the fact that it came on gradually seems to rule that out.

Before I go tearing into the head, I figured I'd get a second opinion. Here's a video of what I'm hearing (especially starting/shutting off the engine):

YouTube video of starting engine

Let me know if you don't have access, not sure if I have the permissions correct.

Thanks!

Exhaust manifold is famous leaking or breaking for VIN R dukes for all cars using it.
get a new one because most used ones are already have problems.

More so when exhaust mount spring etc are damaged or missing. Or have bad "dog bone." See my Cave, Torque Strut

Already has a replacement but could be bad or gasket(s) failed for same reason or installed wrong. 87 & older needed spring bolts at exhaust down pipe joint.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

[This message has been edited by theogre (edited 11-07-2017).]

With one of These, you'd be able to easily track down that noise in less than a minute.

My experience (not exhaustive mind you) is that..
A low end issue has a lower frequency (chunk chunk chunk).
Valve noise has a higher frequency (tick tick tick)
and exhaust leak has a thicker tick (thwup thwup thwup).
Sounds like an exhaust leak to me.
Can you wave your hand around and feel the puffing? Without getting burnt?