Generally they will need a warrant to enter, unless there is some kind of emergency (e.g., a fleeing felon enters the property). Cops can and do get warrants signed in the middle of the night - I remember many a phone call to our house when I was a teenager (my mom has been a judge for about 20 years), asking for warrants to be signed. It's important to understand, however, that if the property resident consents to the police's entry, they have waived any issue as to the validity of the warrant (or lack thereof).