Knocking noise on my 190E 16V

I have a 1986 190E 16V with about 16k mi on a rebuilt engine. The car has been properly service since the rebuilt. I have notice a knocking noise comming from my engine lately and it is driving me nuts. I use a 91 octane fuel. I only notice the knocking noise when I drive under normal condition W/O ac on very hot days +90F. I did a search on knocking and pinging and found some answer, but did not find anything about 16v. Pleaes need help!!

Eddie 16V
86-16v Smoke Silver
European Headlights, backseats
Clear Corners
17inch AMG Monochrome rims/tires
automatic (great car in the LA area)
Sony head unit w/ 10 discs CD changer,amps
2-8inch sub woofers