Drum severe vibration

Today, started up the dryer- had a single pair of pants in it. There was a severe vibration. Last loads ran fine. Took shell apart thinking a broken tensioner pulley or something interfering with belt or drum somehow askew. I have teen girls- I don’t ask them questions about appliances anymore. Nothing amiss, drum, exhaust fan, belt and pulley all fine when rotated manually. Reassembed- restarted. 2 second rumble then all fine. ruble reoccurs for 2-3 seconds throughout the cycle- empty or full doesn’t matter. Thoghts from the collective group wisdom?