Has it by chance been exposed to rain/wash job? If so, there might be water at/on the "mother board". I had that happen to me after a gully washer came up one night-the heater made a gawd awful racket-like it tried to ignite and click whine to a certain degree. I removed the grate and unplugged the darn thing, to make it thru the night. Next morning took the outside cover panel off the furnace and found 'the culprit', water pooling on bottom of furnace floor, enough to reach the soldered prongs of the mother board. Dried it up and put some 1/4 inch peel em stick em weather stripping on the indented foam of the panel and reinstalled. BAM, no more issue. Hope this might help. And like you the furnace was NOT activated, in fact had the AC going....
