I have an older (approx. 1988) small Lawn Chief riding mower with a flat tire. The tire (tubeless)actually came off the rim after I tried using Fix-a-Flat. The tire seems to go back on the rim fairly easily but it's loose. When I push the sidewall in I can see the rim and the tire can be...
Could be one of several burrowing animals but it sounds like the location where chipmunks like to burrow...usually near walls & foundations. A Have-A-Heart trap a little bigger than the diameter size of the hole would probably give you an answer.