Midway,
I hope this is helpful and distracts your attention from how quickly the sound arises. You can easily turn the sound down, and you will hear a smooth running engine. Here is where it becomes fun though
You can create any sound that you would like to replace it with. I have done this with the stall horn which in most any dogfight is buzzing away. Instead I copied a decent engine shut down sound and adjusted the volume, so now the engine sputters momentarily to alert me of the impending stall.
In order for sound to appear later than the actual damage, a new criteria must enter the coad. To make a case for your "progressive" damage sound, possibly the oil temperature guage would initialize the sound sort of like the external wind sound kicks in at certain speeds. While your at it, why not add a buffeting noise when a airplane compresses and starts to shake.
Now, that I have distracted you enough with other possibilities, please note that the sound is not a bug, but your suggestion could very properly become a decent wishlist item.