There are a lot of explanations to this 'problem'... one of them being out of balance wheels / under inflation / new- old tyres. Happens even before gear retraction, esp on heavy weight t/o - higher rotation speeds.
Need to mention though, during the gear retraction phase, anti-skid and all are deactivated. Mostly the sound is from the nose gear and as no braking available there, there is a rub pad to do the job which further accentuates the problem.