Boiler trap gargles when sink emptied

[Dev1872]

Recently fitted heat only and teed condensate in to sink waste after sink trap as per MI's but boiler gargles when sink is emptied, looks like it's piling the boiler trap but doesn't empty it. Never had this issue before, any way to overcome? Can a fanny trap be used?

 

[Ben-gee]

Try changing sink waste to an air admittance one.