It cooled off quite a bit today so we turned off the air conditioner. I was in our upstairs bathroom a few hours later (single story ranch) and heard sound coming from the vent in the floor. I went downtairs and there's a noise coming from what seems to be the trunk coming out of the top of the furnace and the trunk itself is really cold with frost on it. I can't describe the noise but it sounds like something running in there.

Two weeks ago the motor on the outdoor unit went bad so I replaced it myself and everything has been working fine. Also, the filter looked pretty awful as I haven't replaced it in a couple of months.

Any idea what this sound might be and why the frost on the the trunkline coming up out of the furnace? Could an old filter cause this or possibly my motor change?

Thanks