Depending on the Rig, Usually the Fans. On the Rig with 4 air cooled GTS250s running at 70%, that's by and far the loudest on that system. On the other 2, it's the 120mm fans on the Radiators.
As for the Poll question, here's the page from the FAQ. I tried it and can post a poll. if you can't just let Strud or Planetarian know.
How do I create a poll?
When posting a new topic or editing the first post of a topic, click the “Poll creation” tab below the main posting form; if you cannot see this, you do not have appropriate permissions to create polls. Enter a title and at least two options in the appropriate fields, making sure each option is on a separate line in the text area. You can also set the number of options users may select during voting under “Options per user”, a time limit in days for the poll (0 for infinite duration) and lastly the option to allow users to amend their votes.