HELP!! 986 rear passenger side rattle

[myn8m3is8ndy]

I have a 2002 Porsche Boxster Tiptronic
REALLY ANNOYING RATTLE ON REAR PASSENGER SIDE.
I'll try to give as much detail as possible!

After 5 minutes of driving or so, I would get a really annoying rattle in the rear right side when accelerating. It's super embarrassing because it's such a nice car but the sound, just terrible... When I am fully stopped on any flat or up hill ground, and start to accelerate, the rattling is there and when i release off the acceleration, the car rattle fades away. So the rattle is only when I accelerate.

I brought it to a Porsche specialist today and he told me that he wasn't sure what it was and to wait for the rattling to get worse to better pinpoint where the rattling is later.

It did pass smog.
New tires, New breaks, New breaking fluid, oil changed.
Does not rattle when when gassing on neutral or park.
Rattle goes away very randomly and comes back after a minute.
The rattling reflects off of cars. Like if there is a car driving parallel to me, the sound would get louder vs. driving next to nothing.
rattling is still somewhat there when at high speeds but the engine drowns out the rattle.
Today i noticed while driving on the freeway, it was in 5th gear(tiptronic) and i was mashing it, gas pedal all the way and it would take around 1 or 2 seconds to go from 75 to 76, 76 to 77 and etc. Like the car felt slower.

What can this problem be????? I really dont care if the car is a bit slower, but the rattling is driving me crazy........