2009 Cooper S. The sound is intermittent, and I can hear it inside the car sometimes, but the engine is running smooth and making great power with no trouble codes. It sounds exactly like if you were checking spark on a spark plug, but louder. If spark was arcing from the coil to the head, wouldn't it misfire and run rough?

 

i think that sound is coming from the lifters. mostly happens when the car is cold and it is a hit or miss. how many miles you have? if you ever removed the valve cover, what is the condition of the inside? gummy, burnt, or shiny yellowish?

 

It really sounds like the 'snapping' of a spark to me. I pull the cam cover every month or so to check the cam chain guides. The engine is very clean, there's no sludge at all and the oil is always clear. The engine has around 175,000kms. I'll check it tonight in the dark and see if I can see a spark.

 

I never heard of minis making stray sparks. Use engine flush in your next oil and filter change.

 

I don't know that it's even possible to have a stray spark with the ignition system on these cars.
There's no way you could have a stray spark without a poor running engine and a code thrown.
MiniToBe is right, it's most likely your lifters.

 

I wiggled the coil packs and found that the sound was coming and going on cylinder #2. I pulled the coil out and there was brown tracking marks on the side of the coil pack plastic tube. I brought it in the house and cleaned it off, then smeared some thick Dow Silicone grease all over it and re-installed it and the sound is gone. The plastic didn't appear to be cracked, but looks like there must be a tiny path allowing spark to get through.

 

Well done! Any pics?

 

Yes , they can dry out from the heat then arc. Might buy it sometime before the heat cycles dry it out more.

Di electrical grease helps when you first install new ones.