AP400 fan connector..help

I'm trying to quieten down an old Compaq ap400. I've found a much
quieter fan to fit into it but the new one only has two wires from it
and the old noisy one has three. The connector on the motherboard has
four prongs with a gap, something like this: | | | |
The connector wires a
black (gap) red (gap) yellow
Now, can I just connect the black and red wires, or is the yellow wire
a sensor or something?
I was also considering wiring it up to a molex and reducing it to a 7v
instead of a 12v, making it even quieter. Any help would be
appreciated, thanks.

Andy