jms338

I recently took my Forester with 12,000 miles on it to my local dealer for what I believe is engine ping. I was told by the mechanic that rode with me the the noise I was hearing was air moving through the tumbler valve. I am not completely convinced that I received a correct diagnosis so I was hoping someone might be able to provide insight into whether this makes any sense or not. This noise occurs regardless of gas brand or octane level. It occurs on acceleration and when climbing hills primarily, but has also been noticed when cruising at highway speed. I can only describe the sound as a rapid fire knocking noise from the engine compartment. The noise apparently is not loud enought to trigger the knock sensor. The dealer was not able to find any diagnostic problems. Is it possible that this something I am just going to have to live with? The owners manual states that you should not be concerned if the engine knocks occasionally on hills or acceleration, but how severely or persistantly does an engine have to knock to cause piston damage?