Without knowing more, I can think of two possibilities:
1. Something on your motherboard is overheating and causing the fan controller to kick into a high cooling mode.
2. If you have a voltmeter, try measuring the voltage on the fans. If it's high (greater than 12 volts), it would suggest you have a power supply problem.
If Dell's motherboard uses standard power supply connections, you could try connecting another power supply. If the system continues to do the same thing, the problems' on your motherboard or some component plugged into it.