'01 F150 Knocking sound

2001 F150 Super Crew 4.6L 180K miles

Changed plugs
Cleaned throttle body of carbon buildup

I have a slight (my wife can not hear it) knocking sound from my
engine when accelerating from a complete stop. This only happens with
the a/c on AND when the truck has been drove 25 to 40 miles. It seems
(I think) to only happen when the outside temp. is above 90 degrees
because I don't notice it on my way to work in the morning.
The Haynes repair manual says to replace plugs, which I did, and
check the injectors. The injectors seem to be OK since they are
making the tapping noise using a listening tube. The manual also says
it could be ignition timing off a little, the book also says it can
not be adjusted either.
Other than starting from a complete stop it does not seem to make
the noise accelerating when passing or going up hills.
Any ideas?