Air1727 From , joined Dec 1969, posts, RR:
Reply 3, posted (10 years 6 months 6 days 23 hours ago) and read 1585 times:
Pressure differential between the high pressure air surrounding the exhaust and the low pressure high speed air on exit. During high end thrust, the boundary disturbance becomes so high you hear anomolies like popping or snapping sometimes. With higher bypass powerplants, the bypass/exhaust air mixture is more pronounced causing less disturbance.