hilux diesel snorkel vibration

Have you got the foam filter thing in the inlet of the snorkel, you should have if it's non turbo. Make sure it hasn't rotted or slid down to the bottom if it was there.
Also make sure the bracket at the top isn't touching the A pillar.
Or do yourself a favour and fit a turbo;-)

interestingly a guy posted ages ago about fitting a hiclone to his snorkal and the only benifit he got was a reduction in the vibration - that led to a few interested followups rather than the usual negative feedback about hiclones

I see alot of these come through my work shop from car yards as far up as 2000 models and on.
The resignation in the air intake usually vibrates the left front park light loose. For car yard cars, I remove the light and secure it with silicone as it only has one screw and a snap style clip holding it. It only reduces the vibration and doesn't fix the underlying problem.
As soon as I find a solution I will put up a post. I think it is in the air intake ducting up inside the guard. I had a similar thing with my 80 series and when I fitted the snorkel I had to remove the ducting from inside the guard and thats where I found loose screws.
I would try removing the left inner splash guard and having a squiz inside the guard to see what you can find.
Best of Luck

Another thing to note is that if you dont drill the holes for the snorkel mountings as per the instructions, you may get vibration and noise transmitted through the snorkel from the stud rubbing on the fender.