Finally found the source of the smell in my basement. In a hole under the floor, accessible by removing a couple boards, I found an unsealed sewer cleanout. It had a rusted metal cover sitting on top of it held down by a pair of bricks. Looks like the original four bolts are snapped off and still in the holes in the flange. Any ideas on how to seal this? Preferably in a way I can open it in the future if needed. The white in the picture is the concrete foundation.