Police receive an anonymous tip that S is dealing cocaine out of his house. An undercover officer obtains entry and buys a package from S. Out on the street, a drug-sniffing dog alerts to the package. Together with the dog, police, with the intent of asking consent to search, walk up the stairs of S's porch. At the door, the officer hear the repeated sounds of shouts and toilets flushing. May the police force entry, without a warrant?