Thanks for the feed back but perhaps I should give a little history.
I bought the original NUC from a local supplier (Christchurch New Zealand) with a 1 tera bite hard drive and complained about the pitch quite quickly so the supplier changed the drive to a solid state smaller one at my expense. This did not fix so the whole unit was changed under guarantee and I had to deal with Micro Soft for a couple of weeks as they thought I was pirating soft ware.
New machine has the same problem.
Latest attempt has indicated that it is not the fan as my techie (who has been to too many rock concerts and cannot hear what I am on about) has adjusted it so the fan only comes on when required and this is a not unpleasant rushing sound although it is loud and is not the sound that is annoying me.
I have checked every thing external including my sanity. I work in a quiet environment.
On boot up there is a very brief pause in MS office loading and the sound disappears for that very brief moment before coming back again.
I note there are silent NUC'S listed and wonder what the difference is?
I now suspect that it is a standard component in my model that is causing the problem but which component?
Sorry to bug you Guys but more thoughts as I really want to sort this out.
I have realised that I did not have my external power source changed as part of the warranty swap and when I take my unit to my techie he uses his own power source.
Is it a possibility that some thing in my remote power source is creating this pitch?
I have checked. The pitch comes from the NUC not the power source. We have 240volt 50Hertz in this country.
It is hard to tell but at this point to try to isolate this issue you can try removing every component in your system and then add one at a time to find out which one is causing the problem.