This question is asked often. Bumps in the lawn that make it uneven, hard to work on and plain unsightly, could, sometimes, actually be a god thing. From the enclosed pictures it appears that you have an army of earthworms and night crawlers providing”biological aeration” for your lawn. These little acorn-like hardened structures are called ‘castings’ with a tiny hall in the middle that allow air to get deeper down. Thinning of the lawn can also be a sign, among else, of more compacted soil. Earthworms too need oxygen to survive and take some (for many of us) unexpected steps to overcome air deprivation… they chew through the sod and thatch, come to the surface for a breath of fresh air. You may see more activity in spring and fall with increased rain events. The excess moisture will cause them to move towards the surface to get more air. Believe it or not, these bumps are an indication of improved soil texture…below each one of those castings there is about ¼” diameter hole/tunnel left behind.   Now, the question is what to do about it?  Your lawn would benefit from dethatching whether you use the dethatching rake or have the core-aeration done by lawn maintenance firms. After that application, over-seeding is strongly recommended followed by regular fertilization program to keep your lawn strong and thick so the bumps will not be much of a problem.

Thank you for your response.  I think your analysis is correct as I do usually find a hole in the mound about the diameter of a pencil.  I do have one concern though.  You described the structures as "acorn-like".  Do you mean the castings are about the size of an acorn?  Because mine are about 2" in diameter.  Thanks again for your help.   

It was not a reference to the size but more to the appearance of it. The castings almost look like an acorn.