But nobody really explained why.
Slant six engines have solid lifters instead of modern hydraulic lifters that use oil pressure to take up slack in the valvetrain.
Solid lifters require some "excess" clearance in the valvetrain to allow for valve growth when the engine gets hot. Otherwise, when the valves expanded, they would end up not able to close all the way and the engine would run poorly.
So, with the clearance adjusted in, they click. Shouldn't be excessively loud, but they aren't silent. later Slants did change to hydraulic lifters but that was around the early 80's.
The 'get used to it" guy above pretty much made a useless post but he's right, you will be getting used to it.
Air-cooled VW's and a lot of the older Japanese engines had solid lifters, incidently.