I have had the 15 inch 7548 for two weeks now and the fan is very annoying. It would turn on and off at various times. Most consistently, it turns on when I start watching a flash video from the internet. The fan would crank on and then off, and the cycle of on and off would be repeated. Sometimes, the fan would turn on and off even when I am doing nothing and the desktop is shown. I have other laptops in the house and they don't display this behavior. Fan is quiet when I am watching videos.
The fan would normally turn on when I am doing Windows update.
Two reasons why this is annoying: 1) watching a video with the fan turning on and off is aggravating because it is distracting. 2) The battery life is decreased when the fan turns on and off constantly - sometimes I can only get 4.5 hours of battery life because of this issue.
The fan can be annoyingly loud.
Here's my analysis of what is happening:
1) Dell might be using a poor choice of fan in the laptop - loud, and only have discrete increments of speed. It seems that the fans would be on low, med, and high.
2) The thermal window is not set right in the BIOS - making the fan come on and off too early (low temperature threshold - and often unneeded).
I don't think that my laptop is a defective unit. This seems more like a design problem. There are many complaints from people who have the 13 7000 series laptop who are experiencing the same issue. The latest BIOS upgrade for that system seems to alleviate some of the issues. I hope that there will be an update for the 15 7548 system soon.