Mine does the same thing. Its an 02 with 90k. Only does the noise under load, never in park or neutral. It makes the noise through all the gears and only stops for a slplit second when the gears shift.
My car is making the noise on the passanger side, on the valve cover closest to the firewall side. I was told by my mechanic that it is "valve chatter".
I had stopped using synthetic at 85k. I just put conventional oil back in my car, and it seemed to make the noise less noticable, but it is still defenitally there, and can be heard clear as day, especially at night time when there is very little traffic.
I dont know for sure if it really is valve clatter, and dont want to go ripping things appart as of yet, as it seems to drive fine, just has the noise. Hope she dosnt blow up.