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.