What’s that weird sound during spool up on some aircraft?
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.