If the fuel quality is bad enough, and under particular load conditions, there is certainly the propensity for it to knock or ping. The knock sensors will retard timing to a safe level until it is gone. However, going from 93 to 91 explains this as the DME will advance timing on it's own. When you went back to 91, it was expecting higher quality fuel and thus there was too much timing advance.

I'd always run 93 if it was available here!

And if you put 87 in it, you better believe it's going to ping. There is fallback programming for low octane fuel as well as low / hi octane maps in the DME.
The pinging noise has probably gone away because it has re adapted to the poorer fuel quality. These cars have extremely sophisticated engine management systems.

__________________
Mike Benvo
BMW/Coding/Programming Specialist
2010 BMW E60 550i Msport
2008 BMW E90 M3 6SP
2006 BMW E60 M5 SMG (RIP)
2003 BMW S/C 330ci, 450WHP (retired)
1990 BMW 735i Turbo
http://ultimatetune.com/Coding.pdf