Hello again, and thank you for your additional information.
The parasite your vet is suspecting, if he didn't mention it to you, is the larvae of the bot fly, called a 'bot worm'. It burrows under the skin of the cat and then comes up through the skin to make a tiny airhole to breathe. If the bot worm is successfully removed, without crushing it, it won't harm the kitten. If it's not removed or if it's crushed during removal, it releases a bacteria-laden substance, and that's why your vet put your kitten on Clavamox as a precaution.
Regarding the rattling noise you hear when he's purring, he may have some slight congestion that could be related to the beginning of an upper respiratory infection (kitty cold) or he may have the beginnings of bronchitis, asthma or pneumonia. However, the fact he's currently on Clavamox is very good, as it should help to alleviate any infection he may have.
Continue to monitor him for any further symptoms of congestion, sneezing, nasal or eye discharge, coughing, etc., and if the rattling becomes more pronounced, or he seems to have trouble breathing, call your vet to report this. Young kittens don't have their full immunities built up yet, so they're more susceptible to these types of infection.
Running a humidifier in your home, or in the room he spends the most time in, will add moisture to the air and might help. Also, to decongest him, if he does have congestion, run a hot shower to steam up your bathroom and sit in there with him for 15 minutes, every few hours. Wrap him in a towel before coming out, so he doesn't get a chill.
I hope he'll be fine! He sounds like a cutie! : )