I believe that would be the roots blower. The manual that came with Soundtraxx unit I got with these sounds (DSX-something or the other) had a paragraph explanation about it. I am surprised that same paragraph didn't find its way into the Tsunami manual. The manual should be available on-line.
This is one reason I've always prefered Sountraxx 567 sounds over the other brands, most have overlooked this. They also do a "bog down" sound of the prime mover when throttling up which I love and find much more prototypical than other vendors sound clips.
That should be easy to solve. Go though the list of sounds for the decoder and mute each one until the sound in question goes away.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
I have recently installed a Tsunami in a EMD F3 diesel, and am puzzled by one of the sounds. It occurs when throttling down and is especially pronounce dhen dropping to idle. the sound is "rawrrrrrrrr" and to me sounds like a turbo rundown whine, but first generation 567 engines didn't have a turbo! Could it possibly be a generator sound? None of my other first generation locos have it. John Colley, Port Townsend, WA