If it is coming from the bushings, I don’t think that there is anything you can do ( outside of of replacing them with greaseable). I believe the rubber bushing type is standard equipment and grease type are an option that would have to have been specified at the time the chassis was ordered. Has it been making noise since day 1?
Soak with “Super Lube” in the spray can. Grainger carries it. We use it to set rubber and poly bushings in everything. It is about the only thing other than silicone paste that will not damage the rubber.