i have been using double blades for the past 2 months. The other day i went back to single, wavy blades. Today i cut 3 lawns. the first two went normal. At the start of the last one i noticed that my ears were hearing loud, thundering noise coming thru the seat and the machine. I always have earplugs AND ear muffs, so i thought that the earmuffs are going bad and needed a new pair. Reduced the lever for the RPM halfway and the noise was still there...but of course not as loud. The only way to make it stop was when i turned the blades off once i was done. i did not see problems with the cut at all.. it sounded like a sheet of thin aluminum during a storm. could one blade be loose? it was too late and did not check under the deck. i always use a torque wrench when changing blades. The noise seemed to hit the right frequency and was umbearable even with the hearing protection i had on. any ideas? the mower has exactly 100 hrs. i am a part timer.