Q: iMAC i7 3.4 Ghz Making a Ticking Sound. Help

This very well could be a failing HD. FIrst backup EVERYTHING using Time Machine or create a bootable clone.

After you have done that use Apple Diagnostics following the directions in Using Apple Diagnostics - Apple Support

Please disregard my instructions above, please use Apple's Hardware Test in Extended Mode 2-3 times back to back. The reason for running it multiple times is AHT may not find errors after the first or second pass however when it finds problems they are correct. If it does find errors then the next step is to book a Genius appt. at your local Apple Store or visit and AASP for help. Instructions for AHT are in Using Apple Hardware Test - Apple Support

Sounds like the noise only starts when you are doing either processor or graphics

intensive processing. Most likely it is a cooling fan related issue. Either it's wobbly

or it just needs a clean-out. A lot of dust and fibres get pulled into the system.