Microphonics?

This could just as easily be in the audio section, but here goes!  I have a Dell Inspiron 1545 with a problem---if I set the microphone slider mixer control to max, I get an awful howl in the speaker.  If I reduce the intensity, it reduces and eventually stops.  I believe this is a form of feedback as if there was a microphone was live---I was wondering if this laptop has such a microphone inside the laptop? Or,perhaps it is a problem with the external microphone jack on the front of the laptop?  Any ideas, help or solutions?   thanks as usual

Bill in Las Cruces