H'mmm hydraulic lifters in a OHC engine. They do have hydraulic lash adjusters, kind of the opposite thing.

 

Yeah regardless of where the sticking is it’s scary. Even if the valve stem is sticking in the guide, could be gunk, could be bent...

I hate cold starting in really really frigid weather, paranoid about a valve sticking open...

 

Also may be a cracked exhaust manifold. I've recently viewed this problem on youtube.

 

My next suggestion it the flex pipe, a crack in it can cause a ticking noise, then it’ll open up and really be noisy, that is usually caused by a worn out upper motor mount.

 

Might sound odd since you aren't used to it? Ticking is somewhat of a normal sound, clacking and clattering are not.