James, the usual cause of creaking is dirty pads in the hitch.
This can be caused by not having removed all the paint from the towball prior to hitching up, or general dirt from exhaust fumes, or rust which has not been cleaned off the towball prior to hitching up.
Thorougly clean the towball with 200 grade wet and dry, followed by a good clean with either white spirit or brake cleaner.
Next give the pads in the hitch head the same treatment, if you cannot get to them in situ, take the pads out and clean them, then replace them.
Always clean the towball before hitching up, use nylon scouring pad and wipe with lint free cloth and some brake cleaner.