Central air conditioners are meant to seem silent inside your home and to only make a moderate amount of noise when you're standing outside right next to the condensing unit. While the unit isn't meant to be completely silent everywhere, you should still not hear loud noises or recurring thumping sounds.
What can cause a steady thumping sound in a central air conditioner?
Condensing Unit: Debris
The condensing unit has a grated cover that allows for vital air circulation within the unit. The grate holes are fairly small, but debris such as large leaves or small sticks can still sometimes end up stuck inside the holes or make it all the way into the unit. Inside the unit, a spinning fan can repeatedly knock into the debris and make the thumping sound. Leaving the problem in place can cause damage to your fan and then cause your system to overheat.
You should check the exterior grate at least once a month for any debris that's sitting on the grate or that might be stuck in the holes. You can also remove the grate cover and check inside. Make sure you turn off all power to the unit first using the fuse box on the nearby wall. The cover should have a couple of latches or screws holding it in place, and then it will either swing open or lift off entirely.
Remove any debris inside the unit before putting the cover back on and restoring the power. Check to see if the thumping continues, and if it doesn't, you solved the issue.
Condensing or Evaporator Unit: Broken or Failing Fan Assembly
If the condensing unit doesn't have any debris inside that the fan is hitting, the fan itself might be the issue. Check the fan blades for signs of breakages or severe bends that could cause the fan to clang into its own assembly while it rotates. If the fan blades are damaged, call a heating and cooling service technician for a replacement fan.
Fan blades aren't the only part of the assembly that can cause noise. Check that the bearings holding the fan onto its assembly aren't loose or failing. The problematic bearings could have the fan rocking too much while spinning, which causes the blades to hit the assembly and poses a safety hazard. Again, you want to have an air conditioning service tech replace the parts before using your unit again.
Does the thumping seem to be coming from inside your house? The air handler portion of your cooling system, which is located inside, sometimes within your furnace, also has its own blower fan assembly. Check the fan and bearings for damage, then call in a service tech if needed.
For more information, contact a heating and cooling specialist in your area.