I think from th pic (difficult to tell) that these are the same hubs that I have on a box trailer, to suit 10" mini wheels. If so they have a grease nipple fitted, and you should be able to get a complete new hub with bearings cheaper than just bearings alone! Check it out.
However if they are new, they will need adjusting after the bearings bed in, mine did the same after 5 miles with a load and have been fine since. Folow the advice above.
"I'm not lost, I'm exploring...."