Concertina corolla

I HAVE a 2005 Toyota Corolla Ascent bought from a Toyota dealer. After a year the engine developed a ringing-like concertina sound, which you can hear now as the engine is idling. I took it back to the dealer. First they told me the noise was from the auto transmission crown wheel and pinion and not to worry about it because it is like that in most Corollas. I took the vehicle to another Toyota dealer where they replaced the crown wheel and pinion, the air compressor bearings and the water pump, so they tell me -- but the noise is still there. They then told me they needed more time to find the problem. The third time they kept it for four weeks and told me they sent the engine away to a motor engineering shop and it was pulled apart. They couldn't find anything wrong with it and told me they have driven other Corollas and some have the noise, some don't. After all this, they now tell me this is a characteristic of the engine. Could you please help. The noise is very annoying and gets louder as you speed up. The vehicle is still under warranty.