I am making 2nd generation lentiviruses by transfecting HEK cells with the following; pMD2V, pPAX-2 and a lentiviral plasmid vector. I harvest the media at 48 and 72 hours, and then spin (4500g for 15min), filter (450nm) and ultracentrifuge to get the virus  I can see a pellet clearly for all my different lentiviral plasmid vector variants, but all have a brown pellet that is easily dissolvable in PBS, and some have the brown pellet laying over a white pellet that is not very dissolvable and typically sticks to the tube very firmly. Is this just debris that hasn't been removed from the media or is it a contaminant ..... ? Any help gratefully received. So far I have confirmed all my viruses produced, successfully infect HEK cells and produce GFP.