It sounds to me like Diaphragm buffeting, on the side of the carb is a metal plante held in postion by 4 screws, this plats has a circular rim and if you look carefully, a hole in the metal plate. This hole vents the diaphragm to atmospheric pressure.
Note the positon of the hole its different on different models of carb (this is assuming a Walbro carb) its imortant to get the hole back in the same position it came off.
Take off the plate leaving the diaphragm and springs inside the carb and solder a small length of brass tubing or a pressure nipple to the plate, ensuring that the hole in the brass tube or nipple is not blocked, Refit the plate on the carb exactly in the same position it came off.

Either fit a trumpet to the carb and join the new palate nipple to the trumpet,
or simply lead a length of fuel tubing to the inside of the model's fus.

I cannot claim originality for this cure as it came from the late Paul Landers of Just Engines in the UK.

MIke