Wiktionary
n. 1 (context computing English) Spurious characters due to http://en.wikipedia.org/wiki/signal%20noise in a communications link. 2 (context programming English) Data that looks random, as when outputting a binary file literally. 3 (context programming pejorative English) incomprehensible source code or programming language (often due to terse or overuse of operators). Write-only code or a http://en.wikipedia.org/wiki/write-only%20language.
Usage examples of "line noise".
The inquiry soared into a full-blown peak of response, paused as if waiting for a reply, and then sank again until it disappeared back into flat-line noise.
Now wouldn't you think that if she was going to be so devious she'd at least send a cover sheet, and then blame it on line noise?
She slammed the handset down, then picked it up again, jiggling the hook switch rapidly, trying to make enough line noise to sever the connection.