I work with MRI machines, and I know what you are talking about.
Metal to metal is the main source of spurious signals to these machines.
The only other ingredient in this receipt is the presence of high magnetic field.
I am talking about very low signal levels, perhaps 4 dbm or more.
The metal to metal friction causes "white pixel" on the MRI images. These are high energy signals with a very high amplitude and very narrow (very short duration).
Just about any mechanic component in the machine may cause these signals.
They may also be produced from electric malfunction such as high voltage electric arcs (lose connections, failing electronic components such as diodes, or passive components arching, etc).

I guess to accurately charecterize the spark plug ring and their armonics we may want to use a spectrum analyzer.
The scope by itself may not tell you the whole history.