simple, the wah part at the beginning.
Give a "Wah" on each note. Also, on the last note, not neccasery, but it sounds good to
a tiny bend on it. Not even a 1/4, maybe an 1/8th bend.
[ Tab from: http://www.guitartabs.cc/tabs/m/marvin_gaye/lets_get_it_on_tab.html ]
e------------11---13-------------|
B---11---13----------------------|
G--------------------------------|
D--------------------------------|
A--------------------------------|
E--------------------------------
|