Sentence 1: I kept hearing a strange noise in the apartment. 
 Sentence 3: I went to the source of the noise 
 Sentence 4:  It seemed to becoming from the AC 
 Sentence 5:  I opened the blinds to see a group of birds hopping around on the AC 
 Option 1: I was worried that it was mice. 
 Option 2: It was coming from the unit next door.