I have a 1999 Mecury Grand Marquis, 79 K. Light knocking /ping upon shifting into fourth gear , seem to happen once car is fully warmed up. Gradual acceleration no pinging, too the floor sounds like marbles. I did scan, no error code, all passed on obd 2. Decided to read / change plugs - all 7 looked light tan, normal, cyl # 8 cyl last bank right in front of driver- had lightcarbon build. Different than other cylinder. Replaced plugs & put in fuel injection cleaner. No difference. Initially suspected MAF or O2 sensor, but wouldn't that fail on scanner? No error codes, no check engine light. I'm leaning towards this #8 cylinder -- Intermittent coil opening @ hot temp, or drop in voltage? Plugged fuel injector for # 8 ? Also I've read that light pinging is normal for this vehicle. Please help, you thoughts, or ideas or experience with this would be greatly appreciated.