Sentry doing Attack. Odd wind up; fast wind down.

[kswx29]

Sounds like it has a little bit of trouble winding up...idk.
Also has that thing which makes the wind down shorter.
http://www.youtube.com/watch?v=xqfc_jMP0_E

[murrfarms]

The reason it sounds like it's struggling to wind up is because that Sentry is a single-phase unit. That also explains the sudden wind-down, because of its capacitor start system. The capacitors kicked back in, making it slow down abruptly.