Anak saya dulu ada jugak bunyi kahak macam tu around 7-8 bulan, doctor kata it's just phlegm pooling at the back of the throat, very common at this age sebab mereka baru blaja coordinate swallowing properly! A few of my mum friends' babies had the same thing and it went away on its own after a few weeks, especially when baby started crawling and moving around more. Since the doctor has cleared lungs and heart (which is the important part!), likely nothing to worry about, but if it gets worse or baby looks uncomfortable or feeds poorly, boleh check again lah, just to be safe.