A couple of the older posts alluded to these but you seem to have missed what they were talking about - Terms for the same thing: Helmholtz resonator, 1/4 Wave resonator, Branch Resonator
Google Branch Resonator and the flick to the images results to get the idea
This page has some technical ways to work out lengths
http://mkiv.supras.org.nz/articles/exhaust_drone.htm

You could possibly fit a pair above the diff?

With some creativity you could make them adjustable fairly easily