If your transponder is reporting a high background noise level this normally indicates that you could be in an area with a high background noise level or an AIS channel blocker, or more likely, there is a noise source onboard your vessel.
This particular test is required as part of the international standard for Class B AIS IEC 62287. Its essentially there so if there is a blocker or local noise source on channel or on a neighbouring channel you are alerted to it as it will effect the AIS system integrity. It averages the noise levels over a period of a minute so a high level in itself doesnt mean all your messages are necessarily being blocked, but you will notice your AIS recieve performance will degrade.
Recently we have discovered this is often due to tri-color masthead LED's. LED power supply switching noise can end up in the VHF marine band and add a significant amount of noise on AIS channels when these LED's are placed in close proximity to your VHF antenna.
Here is an example plot from one tri-color manufacturer showing the noise generated by the LED before and after they solved the issue, http://ogmtechnical.blogspot.co.nz/2009/06/update-on-rf-interference-for-trianchor.html
You can troubleshoot the background noise level source by using the advanced AIS settings screen (Vision, iOS app) or the status page in vmAIS. Look for high levels of RSSI on either channel. If both channels are reporting high (>-77dBm) it is likely the noise source is a wideband one like that introduced by a tri-color LED or some other electronics onboard. Turn potential noise sources on and off and look for any difference in the RSSI reading (giving the reading a little time to settle its averager each time).