HP tower, about 1 year old. From time to time, the tower emits a very loud fan/whirring sound - not quite as loud as a hair dryer. Nothing appears to happen to trigger this, and usually stops after about 30-40 seconds, but continued recently for several minutes.

There is no loss of functionality while this happens, but I can do nothing to repliacte the problem or to make it stop. what's going on?

If the noise is at times when you are using a high percentage of CPU then it may just be the fan increasing speed to cool the CPU. It may be time to clean out the system. See: Cleaning the Interior of your PC