My old P4 PC sounded like a jet engine
With respect, that has everything to do with poor case design and cheap, small diameter CPU & case fans running at high RPM, probably with noisy PSU and GPU fans as well, and very little to do with the actual power consumption of the CPU - about 85W for the 3.2 GHz P4.
I've built systems with similar or greater CPU power consumption, and they are very quiet, because I selected the components to ensure that they would be.
I'm not denying that it is possible to get similar computing grunt from a vastly more efficient package (e.g. the Atom, just 2.2W SDP!!), and with such low heat output there's far less requirement for cooling fans with associated noise.
SD