I have the newest 21" self-propelled Ego mower--the LM2102SP. I mowed a small stretch of grass yesterday to test it out and it worked and sounded fine. However, when I went to finish my yard today, I noticed that whenever I push the self-propel lever to put the mower in that mode, the mower would make a fairly loud whirling sound--the faster the speed setting the louder the whirling or whistling sound. Is this normal? I did not notice it yesterday.