Cheers GreazyT, I've not whipped an M50 out (yet) but will get some knock sensors for the day- no doubt it will happen soon. My '95 is pinging a bit at the moment- have cleaned the injectors but maybe it was a few tanks of bad fuel. I a tank of 91 octane in it instead of the required 95 (Aussie 'octane', not RON) a while back which upset it big time. A tank of 98 later its behaving better but still I can hear the chambers 'rattling' when on light throttle settings.

I guess if the sensors go, you'd hear the occasional nasty pang (if listening) rather than consistent rattles....