I agree it's very unlikely that you have two faulty filters. The most likely reason is air getting in somewhere. Something else that might be worth considering is that the noise could be the filter vibrating against something in the cabinet which resonates when the door is shut. The latter should be easy to eliminate, just make sure nothing is touching the cabinet and if there aren't any rubber feet on the filter try putting something underneath it like a scrap of carpet or material.
If i were you I'd take the filter off and run it in a bucket to try and work out what's going on. With it off the tank it's easier to check things and shake the filter to get air out.
First check that all the pipework is comfortably onto the plastic intake pipe, minimum a centimetre. If not hairline gaps may suck in air by Venturi efffect when the filter is running. It can be stiff pushing the pipes on so dip the end of the rubber eheim tube in hot water for a few seconds and it'll go softer and make it easier to push onto the rigid pipe.
At the other end make sure the pipe is similarly pushed all the way onto the little elbow intake at the bottom of the filter. Then close the locking collar by hand until it's tight. Don't use tools to get it very tight as it's plastic and may get damaged. The intake elbow (pictured in a post above) screws into the body of the filter so potentially may leak, although I've never seen it and have used several eheim classic over the past 20 years. The other poster is correct that you can undo it and wrap ptfe tape but you risk cross threading it or creating a leak that wasn't there before by doing this. I would leave it alone unless you're sure it's letting in air after you've checked everything else before dismantling parts eheim have screwed together.
Then check the rubber o ring on the pump head. The old versions you had to push it onto the outer edge of the pump head and it could get twisted as it rolled onto the pump head. A twisted o ring is narrower at the twisted part and can leak. However I'd expect it to leak rather than suck in air. If memory serves newer versions have it already installed in a groove on the pump head. Make sure the impeller is sat centrally on the ceramic shaft. Make sure all pipes are not kinked and taps fully open and screwed together correctly, if you have taps on the pipework.
Put the filter together, and run it empty in the bucket. After a few seconds it should go quiet, now lift it and shake it a little to clear the air. It will run noisy again for a few seconds then run with just a slight hum. I don't find them silent but you have to be close by to hear the hum. If it's still noisy look carefully in the hoses and pump body for air bubbles and trace backwards to work out where they're getting in. If there's no visible air getting in you can try running the pump head in the bucket of water like someone else suggested and if it's quiet at least you know it's not the pump that's the problem.
By doing it systematically you should be able to work out what's making it noisy.
