Completely silent means a laptop that is passively cooled, i.e. no fans. Some netbooks might be that way, but I would trust more a machine that cools itself actively and allows me to run something other than just a browser window at a time. Programs mean CPU activity, meaning heat generated. The only laptops that I would be worried about in terms of noise would be a desktop replacement gaming monster machine. Hint: look at the specs, specifically the CPU and the graphics card. If they are high-end, it will be more likely they will have a bigger cooling solution, meaning greater potential noise.
on the other hand, a redundant cooling solution can offer better cooling for less fan speed, meaning less noise.
The key is in the kind of software that you are planning to run on the machine and the specs of that machine. When I push an underpowered laptop to do some gaming, the fan will spin to the max, but same game would only mildly stress a better laptop, meaning no need for max speed of fan.
Good luck with your search, hope it all works out for you.