I don't want to say my car (Peugeot 406 1.8 16V) is noisy (don't get me wrong) but in normal driving, engine sound can be heard inside the cabin during acceleration. However, when I drive in downhill roads and stop before stoplight, the engine is so quiet idling that I can't even tell whether it is on or off!
This is quite weird to me because if the same thing would happen in uphill driving, it would make much more sense because in uphill driving oil gets hot and internal engine parts will expand and tolerances will be near zero. This is why most engines run quieter in uphill driving but I don't understand why my car runs extremely quiet after downhill driving!!
Can someone explain this phenomenon in some scientific or technical terms?