I agree, very odd, to be that low frequency your really in the electrical noise of the system, I'm guessing yours was a V8 with a divide by 4 converter hidden somewhere, as I've only seen frequencies that low when people get fancy with aftermarket ignition control modules, (I've seen at least a dozen V8 vehicles with 2 bastardized V4 controllers running in parallel and voodoo magic linking them together)
Also Pro-tip to the op, you will want my fancy filtering if your looking at the low side of an ignition coil, 300V spikes are common, and forget direct high side measuring, better way is to wrap your test lead around one of the spark plug leads, and induce the signal,