In my experience, when I use my camcorders in auto mode - specifically, automatic audio gain mode - I hear recorded what you hear. It is especially pronounced when there is no noise when the video is being recorded. The camcorders' auto mic gain is doing its job by increasing the audio gain to grab whatever low volume is available to record.
If the camcorder did not do this, low volumes would be very low and the camcorder manufacturers would get complaints that the mics in the camcorders can't record audio very well.
When I use the manual audio gain control in my camcorders, this disables the automatic audio gain capability - so it is up to me to pay attention to the audio levels going in to the camcorder's mics and getting recorded. When there is no audio I want recorded, I turn the mic gain down. When there is audio I want recorded, I turn up the mic gain to a level that allows for good audio recording levels. It also means that the gain control does not set itself so under low audio level situations, there is no "background noise" recorded...
In my opinion, there is no "magic bullet". Many people do not want to deal with manual audio gain adjustment process. As a consequence, the manufacturers have removed manual audio control from most low-end and mid-range (up to about $800) consumer camcorders. This is a money saver (single circuit rather than two and a switch) for the manufacturers and reduces the cost of the camcorder to the consumer (fewer parts, simpler design).
If the auto-mic gain circuit could be designed to react only to certain audio characteristics, there might be resolution for some people, not all. I presume most folks are not willing to pay extra $ for design and implementation of an even more sophisticated audio algorithm that meets everyone's requirements.
When video recording is made under "regular" audio conditions (i.e., music or TV in the background, someone speaking at a normal voice audio level, etc...) I presume the undesirable background audio to which you refer does not get recorded. That's how it works on my camcorders when auto mic gain is used - basically, the recorded audio sounds acceptable when there are good levels of audio being recorded.
There is an alternative to camcorder-based manual audio gain control use when recording, but it still requires your intervention. Assuming the background noise issue is the same as described above, some sort of "manual intervention" can still happen after recording... Import the video to a computer for editing and adjust the audio level using the video editor of your choice. Export the edited video project to your choice of video file with the "fixed audio".
I hope this helps...
Why in the world isn't this problem getting more press? A large percentage of camcorders have annoying ambient noise originating within the camera and picked up by the built-in microphone. Google "camcorder background noise" or something similar, and the postings of a few thousand annoyed camcorder owners will pop up. I first noticed the problem with a Canon HF R10. Many others noted the same problem. Canon denied it and denies it. Others have complained about other Canon models. I next noticed it on my wife's Powershot SD780, and again found that I was not the only one. Finally, this week, I decided to look into the KODAK Zi8 Pocket Video Camera that a well-versed Best Buy salesman recommended. Then I looked at the reviews. There again...and again and again...complaints about internal noise being picked up by the mic. There's no way to work around it, believe me, I've tried everything the techs can come up with. I have no idea what the common cause is...but I'm sure there is a common cause, and someone out there knows what it is. I'd like to know. And I'd also like to know how so many owners can ignore it; and, how and why Canon and other mfgs deny that this obvious design flaw exists.