Engine knock/pinging

I have a 2004 I35. Has a Nissan VQ35
engine with only 80K on it.  The car runs
great. Oil is good, all filters and fluid levels
are good, but when in second gear and
accelerating under a load (uphill or with a/c
on) there is a distinctive engine pinging. It's
not extremely loud and there's zero loss of
power; when I hear it and punch it all the
power I want is still there.  It only does it
between about 2 & 3 thousand RPM,
usually only when in 2nd gear about to
shift to 3rd and under a load. Sometimes
it's does it going from 3rd to 4th gear
around 45 mph, but it's usually only when
I'm between 2 & 3 thousand RPM under
load. I've tried using only 93 octane (used
to use mid - 89) and have added tune up in
a bottle as well as other various additives
(Gumout, Lucas, octane boosters, etc.)

Could it be a bad EGR valve? Or maybe a
faulty knock sensor? Maybe something
with the timing? I'm trying to break down
diagnosis before taking to my mechanic
because I feel it may be something they
chase for hours without being able to
diagnose or fix. It's not a major
performance issue currently but I don't
want it to turn into one because I love this
car and it runs great currently. It's just
annoying more than anything.