It could be a couple of things:
1/Landing gear brakes activate to stop the wheels spinning during retraction, perhaps it could cause a shudder as the wheels stop?
2/ Airflow noise around the open Gear Doors during the retraction. ( a 400 at heavy weight is doing around 180 knots when the gear is retracting and this can cause a bit of air noise )
Nothing to worry about, you should have heard the BAE146 air noise/vibration during Flap retraction!! It may have been really quiet outside but it sure wasn't inside!!
Last edited by nitpicker330; 11th May 2010 at 12:59.