Does the tractor set outside?
If so, maybe rain is getting into the air intake hole, some part may be missing. Many old tractors never see the inside of a barn.
My old tractor has a rubber horn shaped air scoop that fits under the dash then across to the air cleaner. The hood protect it from rain water.
Has someone washed the tractor and maybe got water up in this area with a pressure hose?
How often do you have to check the oil bath for water contaminantation.
This could be a very serious problem if you were to suck water into a hot engine.
Finding the water leak should be a top priority.
Let us know what you find, or post pictures of your air cleaner.