Had to goto MIL house to do some work on the car so I tested it out. First, on the way there I had it clipped to the cylinder head. Undoubtedly, a lot of valvetrain noise. Initial reaction, it does translate internal engine noises like it's supposed to. However, probably best to attach it to the block instead.
While I had the intake manifold off, I went ahead and passed it through the firewall and clipped it to the bolt that holds the stock knock sensor to the block. Then drove home like that.
There was still quite a bit of valvetrain noise but not nearly as much as before. I've never actually heard knock on an engine and definitely have an untrained ear, but listening to it while pushing through the rpm range everything sounded smooth. I have a conservative tune and haven't seen any signs of detonation so I'm not surprised. Still I think it would be good to hear what knocking would sound like through this to verify it picks it up audibly.
Thoughts on the design... I think instead of a battery clip I would simply attach it to piece of flat copper and drill the other end to bolt to the engine block. In that way it would be more secure to the engine and would probably translate vibrational sounds better. If I get a chance to tomorrow morning I'll try to record it through my lap top and post a clip so you guys can hear it too.