If you only hear the clicks during startup and shutdown, I would not worry. Very possibly you hear a PSU noise and HDD sound. If the noise is constant then you may have concerns.
I have a refurbished XPS 8940. When I shut down the computer, it makes a loud double clicking/popping noise similar to what another user posted on this forum last year ([URL] It seems to be coming from the vicinity of the hard drive. I'm wondering if this is normal, or a defective hard drive, or whether it could be a normal sound made by the power supply ([URL] SB
XPS 8940 Makes Clicking Noise