During the speed range, around 40-50 mph, when the throttle is let off of, the engine goes totally silent. I've found that this is due to the injectors not injecting to coast the van down. This makes sense, however, why does the engine sound different when on the throttle in this speed range? When barely any pressure is being applied, it'll get quieter once it hits 40 mph, then get loud again at 50. It also seems weaker and will much more readily downshift. But if the throttle is about halfway down or more before the 40mph mark it'll go up through 50mph and beyond without a change in noise or feel. What could be causing this? This is a 1997 California e350, chip tuned, but as far as I remember it did it before the chip too.