it is definitely a thumping noise.........and it is not part of the engine as it does not speed up or slow down with revving the engine.
I think when revving the noise is less audible.
Performance is not being affected and the noise does come and go.
It is most noticeable when idling and starting from cold.
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as I am sure you can appreciate, it is difficult to diagnose a noise one cannot hear.
common causes of noise like this would be the lifters (more of a tapping though), one of the belt driver components, or the oil pump. usual diagnosis steps are to run the car without the belts briefly to see if the noise persists and to trace it with a mechanic's stethoscope.
if the noise does not change with engine speed, it could possibly be an emissions control pump, so in that case the pump would be disconnected and the car started to see if the noise still exists or not.
the only next step is a proper fault scan and test plan.
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