I am facing the exact same problem ans we are unable to figure out whether these are serum precipitates or contaminants. We tested whether these could be yeast contamination but these appeared to be far too smaller than yeast cells. Since the media isn't changing color we ruled out the possibility of bacterial contamination. In addition, we have observed that in the presence of these particles my cells change morphology and also have a slower growth rate. Please inform us too in case you find a solution to this problem.
This is true for most bacterial contaminations except the most common one: mycoplasma!