Not easy do describe a sound in a meaningfull way, thats for sure..
Well, it doesn't sound like any valvetrain issue, no clicking or ticking as you get before oil-pressure builds and the hydraulic lifters fill up.
It is certainly not something that has happened over night, but I can't really recall when I first noticed it. I had headers installed until very recently, and back then I just assumed that the headers was striking something when the engine was juddering a bit at start/ idle.
It is not a noise that I get anytime else than just after starting before the engine warms up a bit and settles to a steady idle.
I guess my fear is that this could be a bearing/ rod issue, but I have no idea what that sounds like and how bearing/rod noises behave with RPM/ load situation.
I guess the best would be to post a video clip with sound, I'll se if I can get arround to doing that a bit later.
Br,
