Beat frequency. Multi engined large prop aircraft used to throb (sorry). Ever heard a lancaster in the air years ago? Other younger aircraft with large props can do it but it's a deeper noise than the turbofans. The smaller fan blades on the front fan of large diameter engines would explain the higher frequency.
It is particularly noticable from the ground when bigger turbofans are climbing from 2000 feet upwards, as the engines are under high load but the aircraft is moving slow and low enough for the sound to be heard from the ground. It dissappears as the aircraft passes overhead - for reasons explained above by my learned friend Airbuswhiz.
[This message has been edited by buck-rogers (edited 25 October 2000).]