Bowtie,
while trying to diagnose a problem i was having with the fuel delivery system i came upon this little gem of a post. Turns out this might be the best modification i have ever performed on any of my cars. i will be looking into the possibility of performing the same procedure on my other car this weekend.
http://www.lexusownersclub.co.uk/knowledge..._v2&id=152&c=11
in our Rx 300 I too had knocking and pinging during start up of the vehicle, when it was somewhat cold, and usually for the first 15 minutes of driving, then on the highway it kinda disappeared. my problems all started with a check engine light with a dtc code of 01707, i think, which is lean in bank one. something like that. ok, so after i performed the procedure, using carb cleaner spray [dont use brake cleaner] and one Qtip to remove the solid buildup from the sensor wire shown in the diy, the check engine light has not returned. an unexpected result was that the pinging has gone away.
my recommendation is that you try this procedure and see how it treats you. its easy and free. it took me about 20 minutes from start to finish.
good luck!!