Most air filters are directional. They will not let air pass thru as efficiently in one direction as compared to the other. That is why they have arrows on them. It sounds like you may have put the filter in backwards at first, but now have it correct. I think you may have been hearing the hiss of the refrigerant building up pressure in the evap.coil and with the cfms reduced that was passing thru the coil you would have been able to hear it easier.