Popping sound?

[PiccoloTrombonist1]

I recently learned multiphonics, and have been having some fun with them while practicing. Sometimes, however, i hear a weird popping sound when playing these notes (it sounds kind of like popping a pop-it if anyone remembers those?). I was wondering if anyone knows how this sound is even made? And also, despite my username, I am talking about my tenor.

Also, sorry if I posted this in the wrong category, but ik performance is generally where playing stuff goes.

[JanWillemOverweg]

Maybe you describe the sound of the "Schwebung" between the two tones. Google Translate calls it beat frequency in English.
When the played tone and the sung tone get close to each other, you get kind of a bubbling sound. This bubbling gets faster until it's no longer perceived as single blobs, if you move your tones further apart.
You can try playing Bb and singing an f above. Then it's more of an additional humming sound (additional tone). Then move the f upwards to the next Bb and you will notice the beat frequency slowing down. If you play very close to the high Bb there will be only single "blobs" when you have the intonation just off.
This effect for me is strongest, when I try to play the same note in the same octave, e.g. going down from the f to the Bb.

If moving the sung tone independently of the played tone is still difficult, you could try playing them as far apart, as you can sing. Sing a very high falsett and play a Bb - that's usually a good start ...
Then try singing a siren and leave the Bb where it is.
Maybe that was what you meant - maybe I misunderstood
Liebe Grüße
Jan-Willem