You would have to qualify the definition of normal to get an appropriate answer but I'll go ahead and assume you are asking based on the majority of other children. I would say this is normal. I used to hum all the time. Sometimes I would sing too, but mostly hum but then I started playing guitar. While sometimes I still hum, when I am feeling "musical" I usually go grab my acoustic and play for a bit.
Also, it's not uncommon for someone to get a song stuck in their head. I despise having a song stuck in my head because otherwise, once again, I'll start to hum it without even realizing it. My cure for that is to put the song on repeat until I am absolutely sick of it.
In summary, either get the kid an instrument, ignore it, or find out which song is stuck in their head so they can quickly purge it and you can go back to enjoying meal time.