When you play the whistle in a room with a bit of reverberation, you hear a low "whump- whump" noise. It's real, because you can hear it on recordings of whistles too, even in a damped room (Willy Clancy recordings particularly- perhaps a flag-floored kitchen?).

It could be due to the spiritual energy produced by playing Irish music on the whistle, and perhaps someone could develop a spiritual ergometer to measure this.

Alternatively, it could be due to the mixing of the sound with a delayed echo (say 10-50ms depending on the size of the room), and the resulting effect of this on the non- linearities of the ear and the aural perception process.

I'll leave it to others (first year undergrads) to devise those experiments.