Was the pad always flush with the ground, or you never noticed? Typically pads are elevated higher than the ground. ( 2”+ inches ) Those pads have styrofoam on the undersides. A solid concrete pads weight a ton and those ultra lights are popular nowadays.
UltraLite® pads are durable, fiber reinforced cement pads that flex instead of cracking or breaking. They include a textured surface to prevent equipment sliding and are unaffected by environmental conditions.
How old is the system?
Possibly the pad was elevated higher when they initially installed it and quiet, and as time marched on the vibration and weight sunk the pad and stressed out the lineset to where it makes noise now. Even running and securing linesets is an art. The install guides of the outdoor unit usually gives specific guidelines on how to secure linesets and even when it penetrates a outside wall.
Looks like your unit had some prior work on it as one sheetmetal screw on top side is a different color than the rest.
When was the last time you had a basic PM check on your system. Maybe they ( Contractor ) can elevate the outdoor unit and install some vibration pads to see if that helps?
Someone here mentioned that pads sink in over time due to weight of machine and vibration.