Listening to it, it sounds more like a cat hiding under the seat in a small airplane with one of the windows down; plus there are some crackling noises which I've chosen to ignore.
He asked (he's actually a really good friend) if I could make it sound more car-like even though it was already recorded in a real car with a real cat. His chosen recorder (a small point-n-shoot) for the audio really isn't all that good so the audio needed some fixin'.
Soundtrack Pro has a great feature called Noise Reduction which takes a sample or "print" of a background sound like a hum or an AC unit or something and can do a really wonderful job reducing this constant sound while leaving the rest of the audio more or less intact.
It was of little help here since the bulk of the audio spectrum was airpl...car noise so it's on to more drastic and dangerous methods.
Here's the spectrum of the sound After and Before I reduced the car noise.
You can see the yellow near the bottom on the right untouched side.
That's the car's interior road noise. Yikes.
Noise Reduction wouldn't do much here without distorting the important sounds, mainly, those little yellow blobs you see in the green portion: that's the cat meowing about being car sick. The goal is to get rid of the yellow along the bottom (harsh car noise), keep some of the green (car noise) and the cat's little yellow blobs of complaint.
The first step was to run though a light Noise Reduction to just clean up the audio a bit. It doesn't do much in this case but, whatever.
1. Highlight a portion of the clip that has just the noise you want to remove. The longer the bit the better.
2. Process > Noise Reduction > Set Noise Print.3. Command + A to select all (you're applying the "fix" to the entirety of the clip)
4. Process > Noise Reduction > Reduce Noise...
5. Monkey with it until you get something you think sounds okay.
Next up for me was applying the Channel EQ.
1. Highlight the whole clip with Command + A.
2. Process > Effects > EQ > Channel EQ
It looks like this:
The great thing about the Channel EQ is that you can drag around the EQ in real-time while the audio clip plays and refine your selections by moving the mouse up and down. Here you can see on the right the higher sounds have been brought up and on the left side the low rumble of the car has been brought down.
You can sweep with the mouse (button held down) and listen as the EQ does it's thing. That's how I located the rumble and eliminated most of it. There's still room to improve what's being removed but this is a good starting point for my friend to see if this is close to what he doesn't know he wants yet.
Again, you can toggle the button to the right of the volume slider to hear what effect the EQ has compared to the original audio clip.
This is just the very basics of this stuff. There are myriad ways to achieve the same thing, some better, some more complex, some just take longer. I'll post more as my friend hands me more stuff to fix.