2007 I4 cyl 2.0 liter Kia Sportage automatic with 53,000 miles. Noticed a few weeks ago some metallic knocking sounds from the engine when accelerating slowly. Sounds like marbles dropping onto a metal plate. Happens only when accelerating from rest or 5mph till 20 mph. (or maybe it's still there but I don't hear it bec road and engine noise drown it out) Never happens when I take my foot off the gas pedal. It is worst when the aircon is on and car's going up a ramp at 1st gear and gets louder if I speed up slowly while going up said ramp. The manual calls for 87 octane gas and that's what I use. I filled up a few days ago with 91 octane but knock seems unchanged. Other than that, car runs fine and no check engine light. What do you think, guys?