Just how much sirframe noise is generated(as opposed to engine noise) in the following configurations.
1, Clean at 250 kts and 3000' agl and
2, Full flap and gear down at Vref
I have also noticed, that the GE powered 747's are much quieter as they taxi past than the Rolls powered ones. It seems that the exhaust ports on the pylons from the aux numatic hydraulic pumps emit more noise than the engine itself.