I know that diesel sound, and also what this sound is, if my information is correct. My Lancer also makes this diesel sound when the revs fluctuate, well drop 1 or 200 revs, I hate it. I've had a friend tell me that I need oil in my car when it makes that sound. But it doesn't need oil.
I've been told that the sound is the tappets and they need adjusting, just haven't been stuffed to do that yet I guess. Annoying as the diesel sound is, thankfully it's not all the time, but it still needs addressing.