Most times the air is from under the agitator, as the machines fills, an air pocket is formed under the agitator, once agitation starts, a lot of this air is released under the load of clothes, normally thru most loads it will bypass around the clothes and up to the top, sheets act like a hot air ballon and trap the air and they bellow above the water....no to mention once agitation starts your water level will drop about an inch
2 things can be done to prevent, as the machines get near 3/4 way filled with water, oscillate the agitator by hand to remove the air.....or....drill one or two small holes in the base and shaft of your agitator, depending on which one you have, which will not let the air bubble to form......the only machine this does not work for me is the SQ solid tub with the aireated fill during the rinse, bubbled stream of water plunging under the clothes and rising underneath the sheets, almost makes them lift the lid....