Originally Posted by Oliver Enterprises

Caflashbob - It may be the A/C compressor kicking on (and off). When it kicks on, there is a slight increase in rpm to overcome the compressor drag. Perhaps you're feeling that. And, most A/C systems are not supposed to turn on / off like that at idle. It generally represents low A/C refrigerant. However, with our cars, putting it in ECO mode (which I do) may cause the A/C compressor to turn off (at idle, then back on so the car doesn't get to warm) in order to reduce fuel consumption. That's something I don't know … I only speculate. Mine turns on / off at idle. I can't feel it, but I can hear it due to the Exhaust Vents.