what if you don't tell them...

I do not believe that it is ethical to tape a source without telling them. I believe so because they might say something to you that they may not want to be recorded. Also it could not be good for the source because he might say something insulting towards a certain group of people and not want people to know what he said. for example, if you were interviewing the person(source), they would think its a private interview, but in actuality you were recording them. And so later on when the tape goes on the air, the source would most likely be very angry with the interviewer.