Focus 2003 zetec/lr pinging - what to check?

[+Rennie+]

I have this 2003 Focus 2.0 zetec that has a pinging problem even on 98 fuel. Changed the spark plugs. Got a knock sensor but now it turns out that maybe the Australian ones don't have them...

What else should I check? Is there anything else that could cause the issue?

Note it only happens when the engine is warmed up although there seems to be slight hesitation when cold.