500 feet in 18 seconds isn't that fast (around 20mph), especially if the decline was eased into. It would be visible on the altimeter, but "seat of the pants" wouldn't tell you much.
Despite the fix suggested, what you'd actually feel is acceleration, rather than velocity. 500ft descent over 18 seconds would be about -1.2ft/s acceleration.
Which, shortly after takeoff (i.e. shortly after ascending) could very well feel like a continuation of a leveling off maneuver.
Not to mention, in limited visibility you simply can't trust your sensation of G forces. In the absence of visual stimulation, your mind can hallucinate the feeling of movement that doesn't match your actual movement. You can simulate this by standing with your feet together and closing your eyes.
