Sorry for the late reply Rem,
The whistle bubble is played exactly as the ‘classic’ bubble but it’s really about the right hand chop that’s being played while the left hand is bubbling…so it is a bit confusing. But it is for sure a ‘thing’ a certain sound that you will hear on some reggae tracks out there. There is a good example on the curated play list – Tribute to the Marytr’s by Steel Pulse. Give that a listen. I hope this helps!