I made my own "knock ears" based on the Jaycar Super Ear setup.

I use my brain as a noise filter.

Works perfect and picks up knock well before you'd ever hear it without aid.

I doubt a knock detector would pick it up any sooner or better either.

You just need to know what are normal engine noises and what knock sounds like through the setup.

It's sensitive enough that you can hear oil dripping back into the sump after shut down or even lightly tapping your finger nail against the gear stick can be heard through it when the thing is attached to the block between cylinders 2 & 3.