This is an idea I used for another prop that you might consider. Although it's not a knocker, it makes a rattle that is loud.
I built this out of 2x4s and mounted it under an empty wooden barrel. It made a lot of noise.
Its an octagon shaped ring. Inside there are metal bearings welded to spring-loaded thick metal wires. There are four of them in the picture but I think that's overkill, the thing made a ton of noise. Plus, if you remove three of the balls this will give you a nice vibration too. The only bad part is you will need to replace the boards. They wear down fast.
Each wire is attached to a motor shaft underneath the octagon plate. The plate is just a cut plywood sheet. This worked great for barrels, so maybe you can use it under your bridge.