Recommendations for a microphone/recorder to record audio separately from my GoPro?

Recommendations for a microphone/recorder to record audio separately from my GoPro?
Added (1). So I record every time I ride my motorcycle. I want to motovlog and already have the set up for a mic to record my voice figured out. However, I want to record the audio of my exhaust as well. Is there something that will only record sound that I can use to record my exhaust so I can just add it to my footage later with some software? Is there something I can use that will wirelessly record that audio using my GoPro Hero 3 but won't be to bulky or require a different case?

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