Please help me find potential natural causes for the following event.

Location: Basement of a duplex

Location Description: Cement floor and walls. Wood beam ceiling. One door leading to stairway. One window

Scenario: The door is closed but has a 1 inch gap at the bottom. The window is closed. Temperature outside is a standard summer sunny day. There is a bicycle on a stand. The front wheel starts rocking ever so gently back and forth until it gathers enough energy to make a full cycle and spins continuously nonstop. I stop it. With seconds the event repeats.

I put my hand at the bottom of the door to see if there is a breeze entering but sense nothing. I check the window for air gaps but find none.