I just bought a jd gx325 with 645 hrs. But after trying it out for a while I notice that when mower is not engaged the height adjustment knob has a very very annoying rattle. When you put downward pressure on the top of knob the rattle will stop. Any ideas what it could be? Also the belt vibrates very bad and it seems all belt guards are correct. Thanks any helpo would be great. sore-back