My car just started dropping rpms.Rpm use to be at 1000 and now under 500 rpm.The needle also jumps and the a/c makes a loud noise from time to time.Also when I go reverse it makes noise coming from under the hood and it sounds like its coming threw the dash.Is it the belt,the coil or spark plugs and injectors.The battery is 100% and no lights turn on.The car hasn't stalled but does have a little shake minimum on steering wheel. Please any suggestions don't want to be taken in.I hope its not fuel sensor or catalyst.