I recently purchased a display model Toro TimeMaster 30" mower. On my first use I noticed a noise similar to something hitting the underside of the deck. The noise only occurs when the blades are engaged and going over a bump or uneven area of the lawn. It will make a very loud noise if I engage the blades and lift the left rear wheel off the ground. It will not do it if I raise the right rear wheel. It does not make any noise if I engage the drive and not the blades. Any guesses?