Late last year I noticed when accelerating moderately from a stop or stepping on the gas once the car was warm hard I would hear a loud rattling/vibrating sound from underneath the car. Yesterday driving to the show it was pretty pronounced but typically only when I was "under load" accelerating up a hill or into to the wind (which was really strong at times).

For example, running at 60-62 mph up Blewett Pass I was between 2900 and 3000 rpm. If I stepped on the gas to accelerate even a little I could hear the noise. When I got back into town at a stop if I slowly stepped on the gas to accelerate no problem. But, a little more pressure and here's the noise again. Definitely not quiet either if I can hear it over 60 mph above the drone of the mufflers. I can actually replicate it sitting still with my foot hard on the brake and revving the motor. As I pulled into the shop last night the noise seem to come from the drive line tunnel, but other times it sounded like right under my seat. Really confusing to try and pinpoint (of course, a second person will be needed to figure it out).

My dad and brother thought possibly it's an exhaust vibration. Acceleration is fine, in fact, no problems there at all. I've checked clearance and hangers for the exhaust and everything seems fine. I do have a slight vibration still over 65 so am wondering if maybe it could be drive shaft related.

What other ideas do you have to check? As I said it's irritatingly loud but doesn't seem to affect the running of the car.