mk8 1.0 EcoBoost loud engine rattle

[vaiNj]

Hi everyone,
Need some help diagnosing a loud rhythmic metallic sound coming from my 2018 Ford Fiesta Mk8 1.0 EcoBoost (1.0l, 73.5kW).
Here is a video of how it sounds: https://streamable.com/12isik
Context & Symptoms:

• The engine has a very distinct, loud metallic ticking/knocking noise that increases when pressing gas very slightly, but the sound goes away for some time when driving and releasing gas pedal.
• Despite the scary sound, the car drives perfectly fine. I just drove it for 30 km without any issues.
• The engine does not overheat, there is no smoke from the exhaust, and there is no noticeable loss of power.

Important Backstory:

1. The coolant leaked out completely, and the engine overheated, i got an error in my windshield, saying the engine is overheated and power is limited, so i immediately stopped.
2. After the leak, I topped up the coolant and drove it straight to the nearest mechanic. While driving to the mechanic, the rattling sound wasn't present.
3. The mechanic replaced the thermostat. Seem to me that the noise started after the thermostat replacement, but it was quiet at first and I didn't notice it. 4 days passed since thermostat changed, and I got worried because the sound was getting louder. In the video right now the sound is even louder than it was before.
   A mechanic who did a quick look afterward suspected a piston/internal mechanical issue, but given that it drives normally without smoking or overheating, and the noise started exactly after the thermostat job, I'm wondering if it could be something else. Could it be related to the thermostat install, an air pocket in the system, the high-pressure fuel pump (HPFP), or something else common on these 1.0 EcoBoost motors?

Any insights or ideas on what this could be? Thanks in advance!

 

[Handy Andy]

Have you checked the condition of the oil - it's level and is the Coolant in the Coolant tank changing color possibly a darker color?

Any noticeable smells to the exhaust?

 

[vaiNj]

Oil was replaced few days ago. The level is halfway between minimum level and normal level.
Noise was present with the old oil (which had ~10,000 km on it) and sound remains unchanged with the fresh oil.
Coolant is freshly filled after the leak and thermostat replacement. It looks clean.
No unusual smell from the exhaust, no white smoke, it smells okay.

 

[Handy Andy]

The long term effects of overheating, are just beginning - like with this noise.

Overheating is just that - it got hot, even the oil got thin. So if you use something like 5W-20 - then more than likely this is internal engine damage. You can run around with that noise for several thousand miles, or maybe the next 5 minutes. The only real way to know is to have the mechanic pull off the oil pan and inspect that bottom half of the motor.

This may be due to "scorching" of the oil, - which to me sounds like a rod or bearing that got rounded out and no longer is a bearing - oil is falling out of it and in that channel feeding it; building little pressure in that piston, rod and rings of that cylinder to keep the noise down.

The Biggest clue is where you said...

The engine has a very distinct, loud metallic ticking/knocking noise that increases when pressing gas very slightly, but the sound goes away for some time when driving and releasing gas pedal.

This means a bearing - the noise is from metal-to-metal knocking - when the piston compresses and combustion occurs, that rod presses down, bottoming out on that bearing on the Crankshaft that now has little oil pressure available to keep the banging contact from occurring. The reason it goes away is due to the oil pressure build in the channel to fill up the gap that has now formed - so it quiets down. The other journals that are still intact pack oil (keep enough in there) keeping it back into the crank to let it flow down to that rod bearing and float it - but not under load - this is in free rev, not-idle no load condition. Try this when you're driving and it may survive several thousands of miles with fresh oil of a higher Viscosity - like 10W-30- but the inevitable will still happen - possible continuous-wear-resulting-in-inevitable-loss of oil pressure and seizure of the motor.

 

[vaiNj]

Thanks a lot for such a detailed breakdown. That makes sense mechanically and matches with what my mechanic suspected (he also pointed towards piston/bottom-end damage).

In your opinion, is it worth replacing the engine with a used one on this model? Or could the overheating have damaged any other components as well?