I second the use of grease on the ball and the top and bottom of the spring arm sockets, but found that the L-brackets are the real culprits.
These cost about $20 and install in one minute. HUGE difference.
[From Progress Mfg.] The NEW Sway Bracket Jacket was designed to quiet the ride and reduce wear on the L-brackets. Injection molded from high density nylon. Each package contains two (2) SBJ's, one for each L-bracket.
ATTENTION: Utilizing lubrication on the Equal-i-zer L-Bracket, in the form of the Sway Bracket Jacket or any grease type lubricant, may lessen or decrease your sway control by a small degree. Steel on steel is the highest level of friction, steel on lubricated steel second, and steel on plastic third (but also the cleanest). The hitch head can be lubricated, as per the instructions, with no detriment to sway control.