My motor just started making a rattling noise in the motor. At first it sounded like something was hitting the exhaust pipes. I check and found nothing. It's now progressed to a much louder rattling. It doesn't sound like a rod or lifter. After a couple miles it started getting louder. It normally only makes niose when I'm in gear. To help duplicate the noise, I adjusted the RMP down until the clunking came back. This occured at about 700-800 RPM. As long as the RPM's are above 900 it doesn't make any noise and it has plenty of power. Any idea what the issue might be? I know is not anything rattling on the outside. Should I drop the oil pan???? What next?