严格来说，“嗯”才是正确的。但是在网上，用字没有那么严格。很多人在网上打字的时候使用的是拼音输入法，会比较随意地使用发音相同而且相对简单的“恩”来代替“嗯”。 一般来说，像“嗯”、“啊”、“哟”、“哈”、“哎哟喂”、“喂”这些语气词的用字都会有偏旁“口”。

1 ) Yes, 恩is a noun which means something like benevolence or kindness or love or favor or help that others offer you especially when you are in trouble. We Chinese often say报恩which means "return" or "repay". If I have helped you once,and you do me a favor as a return, then it's called 报恩=报答=回报. 2 ) 嗯is an interjective which is the nasal sound used when one makes a response or promise, or to express doubt .

在平时交流时是一样的，但是如果是书面上，“嗯”是语气词，用来表示肯定，书面就不能用“恩”了