Tiny black particles vibrating in my cell culture...Contamination?

[Sze Wah Agnes CHAN]

Dear cell culture friends,
Have any of you seen these before?  If I pay very close attention, with only 
200X, I saw tiny black particles, some spindle shapes, some more round, 
vibrating in my cell culture flasks.  Are they mere particles in Brownian 
motion?  Or some kind of "living" contamination??  Culture medium appears 
clear and cells seem to be growing OK.  Thanks in advance for sharing your 
experience.
Agnes

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