I took my engine cover off the other day. Most of the time I've had my hood open, the engine was off. Today I had it open and running. I could hear a ticking noise. It wasn't the A/C I turned that off. It sounds almost like knocking or piston slap but higher pitched. I think it is the injectors. I tried it with the cover back on and it was muted. It almost sounds like it is coming from all over but more in the back. The cap over the fuel pump did not make a difference. She still runs like a champ. Is this normal?