So tonight I go through the drive thru and I keep hearing this tic tic tic and then I realize it's my truck. So I start trying to figure it out. Truck is idling at 600 rpm but, if I bring the rpms up to 700 the tic goes away but, if I let the rpms fall back to the 600 idle the tic is there. Any idea what would be causing this? Just for clarification when I say tic I mean it's very light and sounds like a sewing machine tic.