Hi John, definitely induction side, the decat pipe seems to attenuate the noise the TB's make at certain revs (2 resonances).
I could hear the same tone (same rev range), with the Cat fitted but way more pronounced without.
Can't determine why but my guess is its pressure wave related increasing sound as air flows into the trumpets, guess the Cat restricted a little and now better flowing.
But.. My fluid dynamics is a little rusty so I may be talking complete little round pink hairy things here....
Whatever it is, it's not mechanical, so I'm more relaxed, it sounds good and runs a bit better without the Cat, more so after Chris's resetting.
Will still try to capture a sound clip at some point.