Why use 'can' in this sentence?

These are best answered by completing the sentence with a subordinate clause or a further descriptor.

1. I can hear a noise when I place a stethoscope to my chest (present tense). I could hear a noise when I placed a stethoscope to my chest ( on or againsty chest are variants; and here we have the past tense / imperfect tense). I could hear a noise NOW if I placed (or were to place) a stethoscope to my chest (conditional tense). ... so choose the sentence form that expresses what you want to say.
2. CAN = to have the potential to be, as already stated. Now do the sentence extension test. Life could be cruel, but it isn't,( i.e. doesn't utilise that potential). Life can be cruel and it sometimes is (fact) Life could by cruel in the olden days (past tense) He could be cruel as a kid = he used to be cruel when he was younger but he wasn't all the time, i.e. he had that potential as a kid but the sentence implies that as an adult he no longer exhibits this trait.