Pajero 6G74 engine swap issue

[Gaurav]

Lately my 1999 Model 2 door (SWB) Pajero 3.5L DOHC started making clanking noise from engine which used to get increase with RPM. Mechanic advise to swap to another engine as rebuilding this might have some weak spot.

After quite intense search (50 odd shops) found one same identical 6G74 3.5L DOHC engine. Lets call this "NE" New Engine and my old clanking engine "OE".

Mechanic swap OE to NE and kept manifold, coil, plug wires, injectors from NE and rest all (compressor, steering pump, spark plugs etc) used from my OE setup. When we first started the engine, it made sharp metal hitting noise which slowed down in 30 mins and disappeared in 1 hrs in idle and as per mechanic this is normal for scrap engine that hasn't been run for few years. Once engine became quiet it develop very strange mild missing with noticeable vibration at engine bay and in cabin along with very mild DHOF DHOF DHOF DHOF (faster) (note: its not frrrr, frrr) noise especially if you standing few meter away from car (like dolby sound of V8 or old tractor) Why i am saying dolby, as its not coming from one specific side if you go close to the car or go under the car, its only heard from 2-10 meter distance. Note its not clanking, grinding, or metal hitting noise. Car accelerate just fine (very slight hesitation, not much), car goes full rpm almost instantly but drives bit rough. Car temperature stays at 45% of gauge all the time. No exhaust leaks, got it checked. No check engine light. Noticed few droplets from exhaust giving good engine sign. While opening this NE, all the covers, bolts, coils etc looks so fresh as this engine is never been opened and hardly been used.

Just to cure this mild missing mechanic removed the NE manifold, coil, plug wires, injectors and replaced wit my OE stuff, we also throw new spark plugs to be 100% sure. But this missing and dhof dhof noise is still there.

While changing above stuff, we did compression test and all 6 cylinders reads 100 PSI give and take 5 PSI.

Mechanic who is working on this from last 3 days saying this engine is faulty and its noise from Valve, Tappet cover or pistons.

Shop who sold me engine, saying its tune up issue and engine is fine. They are still honoring their full money back guarantee.

I am not sure who is right and who is not. Please help and advise. Thanks in advance.