As Banzaiib said its the compressor close to stall, sometimes when the throttles are rigged a little low on the ground idle it will stay like it , most pilots will advance the throttle a bit to make it go away because its loud as hell and very annoying, new pilots don't always do this. I don't know if you seen this video
but I have been meaning to make a version of this with the hornet , bit of a niche market though.