The snorkel assembly is actually three independant parts loosely connected allowing unwanted movement.
The actual oval pipe and resonator box on the left is numbered piece one in the photo. The little round tube and oval in-between piece in the middle is numbered part two. The actual airfilter box on the right is called 3 in the picture.
There are seams between each of those parts, and in a new car the seams are already loose and allow much movement. After some time, those seams become looser and looser. That results in more and more movement and subsequently it rattles. It becomes louder and louder over time.
Normally all three parts are simply clicked together as one, but they will soon rattle loudly due to unwanted movement at the seams. Just grab the snokel and move it up and down to check.
Here is how to permanently and easily cure it once and for all.
When it first started rattling a few years ago, part one and two were stuck together with some black HT-silicone sealant. The same stuff used for sealing timing chain covers, oil pans, etc. If you look closely, you can see that black silicone in the picture above.
Years later it was getting loose again, but this time at the seams between parts two and three. So the entire assy was washed in the kitchen sink with hot water and some dishwashing liquid, and the mating surfaces thoroughly cleaned. This time my left-over silicone was red in colour, so parts two and three were stuck together with red silicone. See below with fresh red silicone.
The entire assembly is now stuck together and nothing rattles any more. Since there is no possible need to separate these parts, they can stay that way for ever, a permanent fix to a common problem.
The picture below shows all back in place.
EDIT 12/2023:
No rattles since, siliconized seams all in perfect order.