It's very consistent when it does it, but doesn't always do it. I just noticed it yesterday. I started the car, everything was normal. After a few seconds I turned the AC on and it started the "rumbing" noise and I could sort of feel it through the pedal. If you rev the engine it goes away, then once it returns to idle it came back. I turned the AC off and it stopped. Then turned it back on and it started again. It was there the whole time at idle while backing out of the driveway, etc. Once on the road I didn't notice it, even at stop lights or the parking lot at home depot.

I'm not extremely well versed on AC systems, but I don't think it was cycling. From what I know, cycling is when it makes that clicking sound(I assume this is the clutch). It wasn't that sound. It was almost like a bearing noise.

I asked the wife how long it's been doing it and she said "since we bought the car". That is not true lol. I've never noticed it, and although I don't drive the car everyday I do drive it probably once a week. We've owned the car for years.