I have a HP omnibook Xe2 laptop, yes it is old but it still has some kick to it. I originally replaced the fan because it became too noisy and made a pulsating sound after the laptop had been on enough time for the fan to kick on for a cooling cycle. I now have replaced the fan 3 times. In the beginning, the replacement fans work great, then they begin to make the same pulsating loud vibrating noise. The power cord of the fan is not a variable, it is out of the way. Any help would be great...

I had thought that maybe there is an electrical/power problem with the cord supplying the fan with power. Maybe there is too much power being sent to the fan and is frying the fan making it sound all loud and pulsate. But who knows...

What fans are you using? Are they the correct voltage? Are they good fans, i.e. ball bearing?