I just bought a BX25 last week & yesterday had to get off it, I couldn't stand that resonance with the pump & the ROPS... So last night I took a can of black pond foam (we're doing a waterfall) I connected about 2' of polyvinyl tubing to the end of the gun & filled both of the ROPS base mounts & then the ROPS itself. It was slow going b/c the foam starts to setup once it's in that tubing so you can't let off for even 10 secs. else you'll need another piece of tubing.
Today I peeled the extra foam off the end of the ROPS & then let it expand some more, an hour later I had to peel more foam off the ends, & let it expand some more, etc. This afternoon it finally stopped expanding, needless to say I used WAY too much, but the more you can fill that void inside the ROPS the better the deadening effect.
End result, is yeah it cuts down on the resonance as a matter of fact it cuts it out almost completely. Problem is the resonance is coming from other places as well, you just can't hear it because the ROPS was so loud. Don't get me wrong, it's cut the noise level down that I can hear the radio now, just don't expect it to be completely gone.
Oh last thing.... Someone mentioned tuning forks in an earlier reply, we use tuning forks in aircraft to disperse the noise energy they're simply bolted to the pressure vessel seams. You can look on the Internet for QuietFlite, this works incredibly well for a passive system. Just a thought, you might get some ideas that way.
Wormwood