Are You using FBS in Your cell culture? If You do, then take some portion of it and look at it under the microscope. Quite often (it depends on the firm producing it) there are some leftover fragments of placenta present in FBS and they look like contamination.

I usually use filtered FBS.

So, the most obvious answer is out.

Maybe You could try this: take the supernatant from over Your cells together with this floating things, centrifuge it at 300g, take the supernatant from over the pellet and culture it in a fresh flask.

If what You see are bacterial conglomerates (e.g., biofilm) You will never eliminate the bacteria by such low centrifugal force. If You will see it growing once again in similar big conglomerates, if You have bacterial infection.

The same with fungi, there are always some single spores, which You will never pellet through such centrifugation.

You can also try to take the pellet, tripsinize it (or maybe better - use Accutase on it as the collagenase in this reagent will not destroy the cells) and look at under the microscope under immersion using 100x resolution. Maybe it is just a conglomerate of the dead or dying tumour cells or the spontaneously formed spheres of tumour stem cells (they do not look like the spontaneously forming spheres in breast cancer culture but who knows) - if You disaggregate it, You could better see what it contains. You can look at an unstained slide and stained with some easy bacterial and fungal stains, e.g., 1% solution of Congo Red for fungi and crystal violet for bacteria (a part of Gram staining). Then You can visualize contamination better if there is any contamination.

thanks a lot. I will follow & reply...