What should an engine sound like?

I took a quick video of it running, it's hard to hear but during the middle of the video you can hear it.

[ame="http://s72.photobucket.com/user/kellyb43/media/Mobile%20Uploads/trimF6318C5E-7812-4965-A5E5-D65D2E68B30F_zps8dd2b3ee.mp4.html?sort=3&o=1"]trimF6318C5E-7812-4965-A5E5-D65D2E68B30F_zps8dd2b3ee.mp4 Video by kellyb43 | Photobucket[/ame]

As you can see I have to get that close to the engine to actually hear it.