My Flakey (RIP) would do this. It started around 1 year and one half when we would go to pick him up. Oh, by the way, don't be hesitant at all, ever to ask any questions. Anyway, hubby and I were so confused. He was our first baby, and this was many years ago. We did ask our vet at that time about it, as it did cause us to be concerned.
Not sure there were behavorilist back then, probably were. We really wanted to make sure it was not a health issue. The vet said, (from what I remember) it was a behavior issue not a health issue. But he didn't reccomend anything for us to do

.
This may not happen with your little guy, but with our Flakey, the growling led to him turning his head towards us, showing teeth

. So, we would just rub the sides of him several times, as to prepare him that we were picking him up. We knew that picking him up was a trigger for him.
Our Flakey lived 15 years, and this behavior never went away, and one time he did nip me

. My feelings were hurt more then my finger.
The only time I was "allowed" to pick him up, was during thunderstorms. Flakey was insistent that I pick him up, during the storms. Which I did. As soon as the storm was over, he made it quite clear he wanted down.
My BIL has a Jack Russel. When the first got Chloe as a pup, I could not wait to see her and hold her

. I picked her up, took her to another room, was giving her loving, and yup, she growled. I told my BIL and SIL, you may have a wee problem here and I let them know what happened. It was time for Chole's vet appt. and at the end of the appt, the doc did highly reccommend Chloe be seen by a behavorilist. Guess she really was animated during the vet visit. Chloe is 6 years old today, she is my girl

and a good girl. My concern was twofold, for Chole and also their children were younger at that time.
Not saying your babies growling will escalate into a worse behavior, but if it were me, I would have him checked out, first by a vet and then take it from there.
If this were to happen with any of my current babies, I would consult my vet, rule out any health issues, and ask for a reccomendation for a behavorilist.
Hope my experience doesn't upset you, but your little guy is still young and could be a very easy fix with someone knowledgable in this area to give you the tools to correct the behavoir.