A couple of years my brother Jim and I (both RVN Vets) were at a kids soccer game at the local high school. I heard a very distant whoop whoop whoop long before it was in sight. It froze me in time for a moment. My brother said to me, "Do you hear that?". I replied, " Yes, a couple miles out heading this way". When it came into sight, we said together, "It's a Slick" (D model - troop carrier). That is a sound you can never forget.
Joe