How to get rid of high level hiss/whine in interview

Question

I am recording oral histories and interviews for my graduate research. I am using Garageband 2, an iMic, and an omnidirectional mic from Radio Shack. The gentlemen I am working with are in their late 80s and early 90s, so these are softer voices, generally accented, and the recordings are done in a living room or on a kitchen table.
My problem is that my recordings all have a high pitched whine/hiss when I play them back, making it difficult and downright annoying to listen to them. I have made sure that the imic input is selected in both system preferences and the garage band preferences. I've also switched to recording the vocal real instrument (only using AULOWPASS, all other effects turned off) in mono rather than stereo to see if that would help (it fixed the other problem I had of my ibook pulling sounds from both the on-board mic and the external mic).
So, what is this high-pitched whine and how do I get it to stop? Is it a mic problem, or is Garageband picking up other sounds I normally don't hear? If so, how do I adjust my levels? Generally it's bad to listen to in Garageband, and even worse when converted to mp3. At times, it almost entirely drowns out my subjects. I make sure the input on the main sys prefs-sound is almost all the way up, or else I tend to have problems hearing the men I'm recording - for another reason the input volume in the track info is always greyed out and set at the lowest level. It's not feedback as far as I can tell, and I'm sure to have the monitor off and mute my output speakers when recording.
I appreciate any help from anyone who has either had and fixed this problem, or any other suggestions.

Answer 1

I am using [...] an iMic, and an omnidirectional mic from Radio Shack.
Not the greatest combination, low cost equipment often gets mediocre results I'm afraid B-(>
Additionally an omnidirectional mic is going to pick up every noise everywhere in the environment.
My problem is that my recordings all have a high
pitched whine/hiss when I play them back,
You can try fiddling with the standard EQ, GraphicEQ, ParametricEQ, and the various shelf filters, but it's tough to make a recording of questionable integrity sound good.
recording in mono rather than stereo
You have to record in mono, you're only using a single mic.
(it fixed the other problem I had of my ibook pulling sounds from both
the on-board mic and the external mic).
That is not physically possible on a Mac, it's one or the other, but not both unless you specifically create an Aggregate device to use both.
So, what is this high-pitched whine and how do I get
it to stop?
You're not going to like this, but, better equipment.
Is it a mic problem,
Likely part of it.
or is Garageband picking up other sounds I normally don't hear?
You mean "or is the Mic picking up sounds I don't usually hear", and yes, that's possible as well. We get very used to sounds we hear all the time, so much so that we cease to hear them.
another reason the input volume in
the track info is always greyed out and set at the
lowest level.
http://www.thehangtime.com/gb/gbfaq2.html#volumesliderdimmed