my car is knocking occasionally at idle from right under the crankcase.goes away after driving shortly. doesnt do it all the time. took it into the dealerships several times told them about the tsb. and they still say they dont hear it that nothing is wrong. even though its loud. they also said due to aftermarket parts on the car their not sure if they could fix it anyway! should i worry about the noise?it only does it at idle, from top of engine, goes away after driving it a few minutes.