Very stiff springs inside + the needed preload on them.
They are pretty damn noisy.
I installed them in my new engine I built years ago, and after about 2K miles, I pulled them cause they were so loud. They are still sitting on my shelf in the garage, back in their original box. (with stickers too

)
They work well...but just to damn noisy for me.
I bought a set of low mileage CompG lifters from ED and installed those.
~F~