Check the timing..
Is the sound a physical rattle or is it pinging? You said that it happens on hot days when you step on it. To me, this indicates that detonation is happening. First, check the timing and make sure it's right. Check out the ignition computer, too, as it has the capability of advancing the timing. Also, make sure you have the right spark plugs...the wrong ones can cause detonation.
Hope this helps!! Let us know how it goes!
87 190E EVO II Turbo