Does a police officer's initial knock on the door of a house constitute an exigent circumstance?

The second question is whether the officers' initial knock on the door precludes consideration of danger to the evidence as an exigent circumstance. Appellant argues that the knock should have this effect, and cites United States v. Curran, 498 F.2d 30 (9th Cir.1974). In that case, officers knocked at the door of a house suspected to be a drug trafficking location, thereby notifying the occupants of police presence. The exigent circumstances which the government argued justified its warrantless search in Curran clearly existed only because the officers had knocked at the door: it was only after the police had announced themselves that the occupants knew the police had discovered them, and that the occupants had the incentive to destroy evidence. 498 F.2d at 33.