THanks for the response, I've been away from the computer for a couple of days so I am just replying now.
I think it is the fan but I am not sure. It is a steady hum. It seemed that after I adjusted everything slightly (fan and all the connections to drives), it stopped. The fan is screwed in but it can move about in place. Does this seem right? I connected the fan using artic silver. It is a fan that is described as being suitable for state of the art processors.
I also seems to stop sometimes, when I move the computer case a bit
Not sure how to isolate different possibilities. I disconnected video and network adapters by disconnecting the cords , which didn't have an effect. I will have to turn off the comupter to disconnect the HD. However since it seems to take a while after startup for the hum to kick in, I guess the best thing would be to disconnect the HD before I go to bed and leave the computer on all morning. If it is humming, I guess I can assume tyhat it is not the HD.
Second paragraph interesting, Nodsu, of courseit I don't understand it at all, sounds (to me) like you are either talking about biology or in Martian code.