It depends on context. e.g., Did you hear that John is coming to town? - Not that I've heard. Did you hear of any news about john cheating on his wife? - Not that I've heard of.

2

Not that I've heard of.

The first one, we often speak the phrase ''Not that I've heard'' as a response to a question. Example: Friend 1: Hey, man, do you know if the Green Day tickets went on sale yet? Friend 2: No, man, not that I've heard.