11.98v is nothing. Any worries about that shouldn't even enter your mind. We're talking about 20mv here. Not even noteworthy. I wouldn't have any concern until it dropped to around 11.80v or so. I ran one pc with a 12v rail of 11.70v for months. At work, we have a lower spec of 11.80v on most equipment power supplies.
So you hear the click when it's shut down, not while running? At first I was thinking maybe something hitting a fan blade, but that wouldn't describe your situation.