Air1727 From , joined Dec 1969, posts, RR:
Reply 3, posted (11 years 12 months 4 days ago) and read 2282 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.