I thought it could have been exhaust when I first heard it bouncing off buildings and bridges but now that I have opened the hood I can tell it's coming from the engine.

After running a few minutes it quieted down a bit but was still there just audible over the engine.

It almost sounds like a steel-bristled brush running back and forth over concrete.

Just by locating it by hear it almost sounds like it's coming from an injector.

I don't believe you can adjust the valves on this engine. Is that correct?