I find two types of clicking noises with Syrians. When I have rudely woken them or surprised them with my hand in their home, you get an angry clicking, which sounds quite harsh, and is a "Go away, NOW". You can often also see some teeth with this type of click, the full on defensive approach!
The gentler, softer clicking is the happy, content sound
Fur-nando chirrups and chirps very quietly during playtime, which is his excited noise. It's really quiet, but I can hear it clearly when he is scampering around my head and shoulders!