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As a security measure, the NVR will beep after powering on if a camera is not detected on every available channel.
You can disable that.
Once it starts up, right click and select "disable beeps". Just make sure that there isn't an underlying problem, like the loss of camera signal on a channel.
Those instructions are for the LNR240 model. What is different about what you are experiencing? Are you not able to locate that option?
Here is a link to the reference: http://bitli.es/HDJMNm