Compressor making weird noise and not building pressure - help!

Hey guys, I've got a Porter Cable 6 gallon pancake compressor that's about 3 years old. Today when I turned it on, it started making this loud rhythmic knocking noise and it won't build past 40 psi. It usually goes to 150. I drained the tank last week so I don't think it's water. Any ideas? I'm in the middle of a trim job and really need it working.

Classic sign of a worn check valve or a bad pressure switch. When you say knocking, does it sound like it's coming from the motor or the pump? If it's the pump, could be a broken reed valve or the piston ring. Also check the air filter - if its clogged it can cause weird noises and low pressure. I've seen that on those pancake units. Let us know more.

I had the same issue on my old Craftsman. Turned out the check valve was shot. The knocking was the piston slamming because the pressure was leaking back. Easy fix, like $15 part. But make sure you depressurize the tank before you start unscrewing anything! Also, could be the capacitor on the motor if it's struggling to start. Does it hum when it kicks on?

Thanks for the quick replies! The noise seems to come from the pump area, not the motor. It doesn't hum, just that knocking. I'll check the check valve and air filter. Is there a way to test the check valve? And yeah, I'll definitely bleed the tank first.

To test the check valve, you can disconnect the line from the pump to the tank and see if air leaks back. But honestly, for the price, just replace it. Also, while you're in there, check the belt tension if it's belt-driven (some pancakes are direct drive, so maybe not). If it's direct drive, could be the connecting rod bearing. That's a bigger job. Let us know what you find.