Top tips for quieting a ride suspension wise..

[TelisSE440]

Find a nice smooth road and run the car at different speeds, in different gears. A rearend noise should change with speed only. You could also try putting it up on jackstands and put the car in gear. If you're brave you could listen around the rearend as the tires are turning. Have someone in the car to run things if you do this. Safety first...

Something not mentioned yet would be the exhaust system. Make sure the hangers are in good shape and nothings rubbing.
Someone had removed the resonators on my 2010 challenger; it was terribly noisy and would rattle the trunk panels at highway speeds.

Thanks BFM Cuda, will do...

[Challenger_Chuck]

These are all awesome suggestions and I am in the process of checking them all.  I just replaced the back shocks and dislike that the bolts are different threads(coarse on top and fine on the bottom, why dodge why?), I also think the access hole through the trunk is a horrible design, but still oddly funny since I've always hated all of those little rubber plugs no matter the vehicle, and one with an actual purpose amuses me.  Anyway.. Ive found a few things like the driver's side door definitely needs a hinge rebuild desperately, and the hood could use a few of the bumpers replaced. Is there any way I can softly shim the door to make it temporarily "tighter". so I can remove it as a potential(while a bushing/pin kit is on order)? And no... Welding them both shut is not an option...

[HP2]

I put subframe connectors in the convertible I have now, and that took care of a lot of it.

This right here. SFCs are not just for high powered race vehicles.

[TelisSE440]

This right here. SFCs are not just for high powered race vehicles.

I'm thinking of putting in my Challenger too in the future, but those cars shouldn't be rattling from the start (45 years ago) to be needing SFCs to make them more rigid and stop rattles?

[YellowThumper]

I think about the flex in these cars like an earthquake. Any and all flexing will reverberate thruough the rest of the car. Be it additional flexing or just rattles. My 74s body is still original with vertually no rust and no undercoating. Thanks California.  Car doesn't have any rattles but I can see and feel the car flex on some hard turns and acceleration. If you have a numbers matching over the top desired car I would leave it alone. Other than that I would add stiffeners. 
I believe the more details added to these to improve the platforms will always help their value. They will be added to mine when body is done.

Mike

[dakota]

Great comments all. These will be very helpful when I finally start putting my car back together. 

[HP2]

I'm thinking of putting in my Challenger too in the future, but those cars shouldn't be rattling from the start (45 years ago) to be needing SFCs to make them more rigid and stop rattles?

Our E bodies are put together with 1000s of spot welds. All the gaps between spot welds allow flex points. The more rigidity you introduce to the chassis, the less loading each of those spots has to carry. Similarly, welding up seams in the uni-body will produce the same result. The problem with that is it is hugely time consuming and can't be done on a car that is completed and painted, so sub-frame connectors are the less intrusive way to accomplish this purpose.