This particular DIY relies on a factory knock sensor and taps into its signal. I use my laptop amplifier to listen in.
Another DIY that includes information on building one with a microphone -
http://www.autospeed.com/cms/article.html?&A=0353
If you do not have a factory knock sensor the mechanical one brad mentioned also works very very well and likely would be easier than building one with a microphone unless you already have the audio electronics skills.
You can get a stethoscope and tubing routed to a 4" copper tube with 2" flattened and a hole in the end for bolting to the engine.
The intake may work but attaching to the block would be preferable.