Are all the injectors working properly?
I had a similar problem with a 20v, with weird rattling @ idle, when I revved it up a little bit the rattling disappeared (Bad alternator charging at idle)
You probably already did this but, doesnt hurt to double check: Check injector resistance (For 20v injectors its around 3.00 ohms) and voltage (12v with ignition on), for good measure check voltage at the battery when the engine is running, from my recollection the injectors need 12,8 volts or you`ll have a problem.