I have a 2002 ford f-150 lariat and Im hearing a slight grinding noise whenever I get right at about 2000 rpms but after that it cuts out and I can hear it again about 2300 rpms and then i dont hear it anymore. I only hear it in drive I cant hear it in neutral so that makes me think it has to be something in the trans or the rear end. also one of my buddies said it could be the exaust heat shield shaking when the engine twists. I thought it could be a motor mount making it twist that much or even a transmission mount since it only does it in drive. Also, I noticed, right under the valve covers look kind of wet and Im a quart low , so could it be a valve squeaking or ginding when it gets fast. I just wanted to see if anybody had any insight on what it could be.