I didn't notice the transmission shifting related to the noise. I think if the AC clutch were locking up the belt would squeal. I suppose the noise could be the the engine torquing against the mounts, as if the transmission downshifted but the engine definitely is running on less than six cylinders just as the noise occurs.
I had been readying the car for the wife to drive. It had been running with a miss for a long time so I changed plugs and wires. It had a serious valve train tick so I included a quart of Rislone Engine Treatment in an oil change. The tick would subside, come back loud, subside until one day as I was driving down the interstate I decided to clean out the heads and in a moment it smoothed out and ran perfectly. No more tick. So there had definitely been a sticking lifter.
I changed the oil again and included a quart of Restore. It ran great. I finally got everything fixed. The wife started driving it. She was excited that it ran "like new". Then "kerchunk".
