The fan in my Nvidia Geforce card is making a humming noise. Is it ok to use WD 40 on it, or no?
I wouldn't advise it.
The fan of one of my older computer use to make a humming noise. It turns out that it was slapping (almost cutting) some power cords that leaned a bit to far to the right.
Make sure nothing but the card it touching the fan. DW-40 on it... it may not be useful should any of it hit the wrong place, even using the red dispenser. If you know for a fact that you can get the fan, and nothing of the chip, then you could TRY, but I don't think that's going to resolve the problem at all.
WD-40 is not a lubricant as much as it is a water-displacement agent. Besides, the fan in your video card could be loose, or dirty, or a number of different things. I would have to actually look at your fan to help you adequately, but I don't think WD-40 would fix your problem. Besides, the humming noise could be indicating a larger problem with the card itself, which would need to be addressed rather than covered up.
Only do it if it's driving you crazy and you have no other alternative.