I have the 2.0 manual. On a couple of hills around here it started pining. I was at 2000 rpm in 6th then down shifted to 5th, heard the pinging again and went down to 4th. I am not sure if the pinging is normal valve noise, but it sure doesn't sound like the engine likes it. I will try 91 on my next tank.
Pete, assuming your gas wasn't bad, I believe what you were hearing was the high pressure direct injection. At low rpm's I can sometimes hear a pinging/ringing noise but that is normal operation of DI. If you remove the plastic engine cover, it will be slightly louder. If it were actual detonation the knock sensor would have immediately put the engine in a fail-safe mode and the noise would not have persisted.