A power supply fan became noisy occasionally?

I have a Dell XPS 410 computer which was quite for the last three years. Beginning from a few weeks ago, the power supply fan became very noisy SOMETIMES when the computer was powered on. If I turned off the computer and turned it on again, the fan would become quite for hours or days. So it was random. I... show more I have a Dell XPS 410 computer which was quite for the last three years. Beginning from a few weeks ago, the power supply fan became very noisy SOMETIMES when the computer was powered on. If I turned off the computer and turned it on again, the fan would become quite for hours or days. So it was random.

I installed Speedfan but the program could only detect the HD temp.

Any suggestion? Thanks!

Besides Ebay, is there any place to buy cheaper Dell power supplies? I know they do not use ATX.
Update: Thanks very much for all the help! It is good to know that Dell supports ATX PSU now. Does this mean that it is possible that I can use the power supply pulled from another mid-tower Dell computer which is not XPS? The nearby university has a surplus store. They may sell Dell power supplies. I will take a look at... show more Thanks very much for all the help! It is good to know that Dell supports ATX PSU now. Does this mean that it is possible that I can use the power supply pulled from another mid-tower Dell computer which is not XPS? The nearby university has a surplus store. They may sell Dell power supplies. I will take a look at the Newegg deals. Thanks again.
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