What is the test for warrantless entry without a warrant?

We have not upheld warrantless entries where agents lacked an objective basis--other than their own actions unnecessarily alerting the defendants to their presence--to believe there was an urgent need to enter the premises without waiting to obtain a warrant. Thus, in United States v. Reed, 572 F.2d 412 (2d Cir.), cert. denied, 439 U.S. 913, 99 S.Ct. 283, 58 L.Ed.2d 259 (1978), we ruled that there were no exigent circumstances where the officers had made purchases from the targeted defendants some 2 1/2 months earlier and had had no contact with them since: