What’s that weird sound during spool up on some aircraft? Some engines seem to make this characteristic “wailing” noise when spooling up. A good example of such is from 0:50 to 0:54 in the following video: 
                
               . I’ve also heard it on a300s, 767s, and md-11s. Does anyone know the source of this noise and why it only occurs with certain engines?
That noise is almost exclusive to turbo fan engines.  If the air is right, turbo shaft engines can do it.
  On sudden and high power changes the big fan on the front is spinning at a different speed than the motor compressor.  Since the front fan is also the first couple of stages of the engine compressor, the air doesn't move cleanly between the two when they are not at the same speed, and make that sound.
The Navy S-3 made the same sound on power changes so it got the nickname Hoover.
@Rob,if you are close enough to it you can feel it.