I've experienced this, too. It is very very very faint. So faint, in fact, that I thought it was my mind playing tricks on me. I recently switched to a new LG 6100 to see if it would solve a different problem I'm having. (When I'm out of my home area, the caller id doesn't show up until I answer the phone; the new phone didn't help. It's a software glitch on the 6100 that hopefully will be fixed soon.) This new phone doesn't seem to have that problem.
I recently purchased the LG 6100 and every 30 seconds or so I get a mind numbing, faint, but high pitched squeel that just lasts for a fraction of a second. Does this happen to anyone else? Its driving me crazy. Great phone. I just wonder if its this particular phone or the entire 6100 product line.