Thanks to all above for your replies. I will check the fan belt when I remove it as I want to rule out the alternator and water pump although I'm still convinced it is more of an air leak on the induction system. The air intake hoses, air filter element and filter box are all standard factory fitment (ie round plastic filter housing). Idle speed has not changed. The EFI is flapper type. My next job is to physically check all of the induction connections and plenum chamber for possible leaks. I will attempt an audio recording
