Dog Hacking Alot?

my dog is a Chihuahua/corgi mix..i dont know how to describe really what hes going through its happened before a few times a year randomly but since last night its been pretty frequent on and off..its like hes hacking trying to get something up this has happened for like 2-3 years now but hes having it alot since last night..its not a cough the only way i can describe is like when you try to get phlegm up..hes also trembling when this is going on and his tail is down..hes recently had physical and all is well..any ideas what it can be and how we can help him..i was thinking it might be some sort of allergy or hair or dust causing it..if it is anyway we can help him?