Kia thumping

I own a 2005 Kia Optima 4 cyl with 39,600 miles on it. Lately it has developed a thumping sound on the front end. It’s very pronounced between 20 and 30 mph whether accelerating or slowing. It’s rotational in nature, that is, increasing in frequency as the car accelerates and decreasing when slowing. I thought it might be a lump on a tire. None found. The thumping persists even after rotation of the tires. Any help would be appreciated.