Unless you consider yourself skilled at opening up electronics and are happy to own the risk of it going wrong, your best bet is to return it and get it repaired by the official repairer. Don't randomly fire air duster cans in through the vent - these have a propellant which can leave you with more problems than you started with if that gets on your optics.
It is often claimed that DLP has a sealed light path, but there seem to be more posts on here of people with contaminants in the light path on their DLP units than LCD projectors, which were always the typical sufferers. This might be a function of there being more DLP sales than LCD, but my own reckoning is that the sealing on low end units typically sold is very minimal indeed. I've taken apart a low-end Benq before which was absolutely filthy with dust on the DLP chip, and that certainly had very little to avoid dust around the DLP chip.
If you turn the PJ off and shine a torch through from the outside in you might be able to make out what / where it is.