Mad, make sure that "rattling noise" is not really premature detonation (AKA: ping or spark knock). With that much advance on your distributor, you may be causing the fuel to ignite too early and you're causing detonation before the piston has reached TDC. If you have a vacuum advance distributor, pull off and plug the vacuum line, run the engine to about 2500 rmps, then adjust your timing to about 28 degrees. If you're running less than 10:1 compression, regular pump gas and that advance should be sufficient. Some things to try first is simply retard the distributor a little and add some high-octane fuel (92 octane will do). If the "rattling" stops, you know its due to the advance on your distributor. Also, put a timing light on and change the rpm of the engine (don't forget to remove the vacuum line to the distributor). If the timing fluctuates more than 2-4 degrees, you may have a worn distributor gear or timing chain.

Why you ask would this be different under load vs revving it up in the driveway. Well, a little complicated, but here goes... In a free spinning engine, there are different dynamics on the rotating parts. The rings, pistons, rods, crank flex, are all contributing factors. Under no load, they are free to spin and are generally in the same position throughout the rpm band. When under load, the parts position themselves a little differently and may be just enough for the combustion to act differently. For example, when under load, the crank flexes a little causing the piston to be a little further down in the bore when the spark plug fires (as opposed to when you revved it up with no load). Thus, the timing has changed...you now have more "effective" advance because the piston if further from TDC when the spark fires. Maybe giving you the equivilant of 35 degrees advance instead of 34. Maybe just enough to cause ping.

Another option is to take the hood off the car and strap your neighbor to the fender and have him put his ear in the engine compartment to see what is rattling. Just kidding

Best of luck...