I'm pretty sure you're feeling the overload leaf engage. When you are unloaded, it'll never touch the spring pads. If you have a really heavy load, it stays engaged and you also don't feel the jolt of it smacking the spring pads. Moderate loads are kinda the sore spot where you have just enough gap the overloads aren't engaged, but they smack the pads over bumps.
2 solutions (if this is what's happening): 1) Remove the overloads. This is a good solution if you'll never be loaded much heavier. 2) Install Stabil-Loads so the overloads are fully engaged.
https://www.torklift.com/rv/stableload