I just found out what sometimes produces a high CPU load on my machine resulting in a loud ventilator spinning which makes me nuts! This takes place even if I do nothing on my machine, just having some programs running including IBM Notes.
I noticed that the nlnotes.exe produces up to 25% of the whole CPU load when this happened. And it came in waves every few minutes. I then noticed that my built-in Sametime client in the sidebar said it is offline. I am usually connected to the EULUC community but when the problem occured it always said "Connecting...". So in this case the server was not reachable for some reason but Notes always tried to connect again and again.
So if you experience similar problems check your Sametime communities and if they are available. I now switched back to "not connect automatically" and now there is SILENCE again.