Could it be Pinking?

[Bob Culver]

Few moderns have heard pinging. Here it was also called "bottling" because it sounded like some one shaking a crate of beer bottles. My car at 7:1 used to climb at full throttle and revs in the gears for about 2 miles evry afternoon on my way home. A short flal at the summit was fowed by a sharp upwrd turn so had to back off momentarily in 3rd. When planted again the unburned oil, caused a short burst of pinging (and run on). Prolonged pinging however faint can be very damaging. Brief pinging is mainly a  cause of broken rings, although malleable irons very resistant.