I finally decided to try and diagnose this noise in my 2nd gen today. At 1500-1700RPM there's an annoying buzz just behind the driver seat near the pillar. After alot of playing with it I found out that particular RPM caused the sheet metal on the pillar between the seat belt retractor and roofline to resonate. It could be felt and stopped by holding that pillar. I took a hammer and rag and whacked the metal near the rubber seal just enough to change the shape of the metal and no more vibration/buzz. Just thought I'd put this up for anyone who happens to have a similar problem. I've also got a buzz on the passenger side now but I found out that one is from the bed touching the cab. Gota get a long bar in there and straighten it out til I can get a new bed.