Can anyone help please? When I was mowing, I ran over a largish stone. The mower made a noise like the stone was whizzing around with the blades then white smoke came out of the exhaust. When I stopped the mower, I tilted it up (only slightly) to see if the stone was still underneath, didn't see it but saw score marks where it had been. The blades still move OK. When I started the mower again, it sounded like a tin can and there was a bit of oil coming out of the exhaust. Have I done serious damage? What should I be checking next? The mower is only 2 years old.