My air handler seems to rumble in the attic when the outside temp reached 50 or lower. When heating the house the air handler rumbles a few seconds then stops, then again in a few more seconds. It continues till the house reaches the desired temp and the unit shuts off. I opened the side panel on the handler where the fan blower is and immediately the blowing pressure in the house got much better. I put the side panel back on and the blowing pressure in the house went back down. All the colis are clean and free of bebris and the filters are brand new. The air intakes are not blocked, they have good intake pressure. There are no obstructions of the ducts and none of them are kinked or bent. The squirrel cage blades have a little dirt on them, but nothing to make them off balance. I was told by a tech to possibly change the capacitor. Could it be that simple or what else might be causing the runbling? Please help, the noise is annoying. Sounds like a truck pulling up outside. Jody

Sounds like something in the blower area. Blower cage or motor a possibility. The motor could be growling so check capacitor.

Check your system air filters also since by removing the panel and improving the air flow shows a restriction in the return air.

I've changed the filters and even let it run for a few minutes with out filters and still made noise. Blower cage and everything in there is stable, nothing loose. No other blockages. I will check the capacitor. Thank you, now I have a starting point.