Bell UH1 10' above the roof @ 120kts.
if that doesn't do it, double the mixture.
generally gets the windows shaking, wakes the babies etc.
Train horn run off bleed air (maybe, it would depend on the psi required, UH1 will push out 60-70 psi max, generally at 50lbs tq will produce less in the cruise, or even at 120kts), Could go just an electric one, hit the horn as you are passing over the roof top.