I have been trying to solve this deep rhythmic knocking noise since I put this engine in back in 2021. My dad says it's an exhaust leak but I am not convinced in the slightest. The knocking has gotten slightly worse over the years and you can feel it very strong through the floor pan. Engine and transmission performance is normal. Knocking is present at idle in neutral so it's not drive line related. This is unrelated to the rod knock as I replaced the rod bearings early this month with no change in this exact knocking noise/vibration. From the engine bay, the engine sounds great. I am beginning to wonder if this is something transmission related like a badly worn out bearing or pump? Or perhaps something I did wrong on reassembly of the torque converter/flywheel? Could a loose engine mount cause this(engine does not jump when revving in neutral)? I am out of ideas.

(Edit): The knocking does get faster with rpm increase and does not get better/worse with higher idle.

Here is a link to the video taken from below the steering wheel, in front of the gas pedal where it's most noticeable:
https://drive.google.com/file/d/1vrr...ew?usp=sharing

 

Have to mention that I can also feel it though the steering wheel and more intensely when idling in gear. Could just be lower revs causing the vibration to worsen as it idles down to about 550RPM when in gear.

 

Now that I am writing this out I wonder if it's a combination of a bad misfire and an exhaust manifold leak?

 

It does sound like an exhaust leak. Its not metallic, so not something that is super critical

 

Would you say the corresponding vibration could be a misfire or something else? Felt through the floor pan and steering wheel. At idle/in gear is considerably more prominent.

Coils and plugs have about 30K on them. (5 years ago)
Wires are ancient and were bought off ebay as a return item like 8-9 years ago (Bosch brand allegedly)
Fuel pump, filter, and regulator have about 12k on them. O-rings on the injectors are about 9 years old.
Injectors are ancient, OEM original.
Air filter is clean

I am reading a high idle vacuum of 25in/hg. This could be the culprit but I'm not sure if this is normal for this engine seeing as none of my FSM's have this data in them.

EDIT:
Vacuum reading is what the ECM is telling me, I will put a gauge on it next chance I get and see what it says.

 

If it idles at 550 RPM, you don't need a missfire to get significant vibration. The vibration would be pronoynced just because of low idle speed. If I'm not mistaken, the normal idle speed is 800-900 RPM. Do you get the "missfire" vibration if your give it some gas to bring it the idle speed to that level?

 

The vibration is persistant even at higher rpm and matches the cadence of the knocking/exhaust noise, I swear I can even feel it while driving but that's not proven.

I am going to try and tighten up every exhaust component tomorrow. I have done exhaust work on this vehicle before and eveything loosens up and starts leaking again within 48 hours no matter what I do. Everything on the exhaust system is warped and worn out except 02 sensors and the cat. (Admittedly, all work was done with my previous engine, no work on the exhaust has been done since)

I may take this truck to an exhaust shop for a quote because it's beyond my capabilities to fix this exhaust system correctly.

Edit: Heh, what a coincidence. I just posted this reply and almost two hours later, ecm threw a P0300 at me after a hard acceleration. Multiple misfires detected. Guess I have one of my questions answered.

 

Check both manifolds for possible cracks. They are cast iron material and do develop cracks over time. As far as everything loosening up, make sure to use new gaskets, new bolts and new nuts. The nuts that go on the exhsust studs are special type that prevent loosening. But they loose that ability after multiple reuses