I own a 2005 Subaru Impreza 2.5RS Sedan. I have a very loud ticking or rattle noise coming from under the hood. Sound only occurs when gently accelerating; around 2500 to 3000 rpm, then levels out and goes away at higher rpm. Rattle intensifies when car is under load, going uphill, and when using the air conditioner. Occurs in most gears from 2nd through to 5th.
Car must be driven to hear rattle, because at a full stop while car is idling in neutral, engine revs perfectly clean no noise ticking or rattles through all revs. Noise only occurs while the car is in motion.
My first thought was engine “ping” due to low octane fuel. I ran premium fuel for a year, but it did not help. Noise has been an issue for a long while and increases each year. We have had this checked before but the problem is never found. I just want to make sure it is nothing serious.