This is the type of hanger that I used, it just doesn’t allow much movement.

If the hangers use rubber or urethane to isolate the system from the body that's about the best you can do.  Those flex connectors aren't going to be any better.  What those are really designed for it to allow sections of the exhaust to expand and contract.  The flex section allows that expansion and contraction to happen without the pipe actually changing length, instead it changes diameter.  I have two in the upipe from the manifolds to the turbo on my 6.0 Diesel.  They aren't going to isolate any vibration in that location as everything they connect to is rigidly mounted.  They are there to allow for the fact that those pipes can see temperatures from ambient temperature to north of 1,500 degrees F.  If there was no section that allowed for expansion and contraction something would crack.  In fact, the factory setup used a rigid bellows that did crack, leading me to replace it with the modified version using those connectors.  Works like a charm, 150k miles and zero issues. 

Think about it this way, the engine and trans are isolated with mounts.  If the exhaust is also isolated with mounts a flex coupling somewhere in that isolated system isn't going to effect anything. 

If you are getting a drone at highway speed or something like that its most likely the mufflers.  Resonance typically is not improved by isolation or allowing movement.  OEMs use strategically placed weights to damp the frequencies causing it.  This is why sometimes drivehsafts have a cast iron ring bonded to them with rubber, or a rear has a weight hanging off it, or the tailshaft of a trans.  Its essentially impossible to duplicate these canceling devices in a non stock application.  The testing equipment the OEMs use is millions of dollars. 

I have four of those as well when exhaust was installed a few years ago by a local exhaust shop.
They just used screws into floor for muffler location and one of the bolts for rear tie downs.
You may want to check your anchor points.

Yesterday, I noticed that 3 of 4 screws were slightly loose.
Since I don’t weld yet, this was my solution.

I cut 2 pieces of steel slightly larger than the hangar plates, drilled and tapped 1/4-20” holes ( Mine were 1” centerlines), 1/4-20”x1” bolts ( what I had on hand), washers, and 1/4-20” Keps nuts (nut with attached star washer), and blue Loc tite.

I did have to drill out the holes for the 1/4-20” bolts.
I don’t know how much noise this made, I was more concerned about the exhaust tearing out floorboards.
My floor carpet has the Mass Backing (rubber) and new underlayment as well to dampen noise.

Hope this helps

My mufflers are hung under the rear seat.  Holes and plates as Nos681 shows.

Then I have a tailpipe hanger on either side.  I fabricated the mounts for those where they anchor to the framerails, because I needed adjustment left/right and up/down so my GT tips hit the valance openings just right. 

The word resonance may have been the wrong description, It is something I feel NOT hear. I am (like several) chasing a drive line vibration as well that could be to the issue. I have determined my pinion was way too far turned UP, which I have changed. Before I put it back down on the ground I am checking everything, and the exhaust just seem very rigid.  

I developed a similar "feel" when I switched from stock to Ron Morris adjustable engine mounts. They are far more rigid than the factory offering and transmit a lot more of what you refer to as "feel" into the body structure. I eventually adjusted to it and don't even notice it any longer.

Rudi wrote:

Sorry no flex connectors here, thought of putting them though if needed.
Except for the AOD modded Tri-Y headers and home brew "H pipe my exhaust system is stock "GT". which is bolted solid to factory pipes to the resonators to  transverse muffler behind the axel then tail pipes to chrome dual tips. It was assembled loose and aligned,  clamps etc. snugged, checked for rattles and then torqued tight. All stock hangers and hardware incuding the pipe clamps, nothing is welded.
It has a nice rumble at start up and idle and no loudness  at cruising,  totally devoid of drone.
 The whole package (sorry Sal different package) has stayed tight and leak free including the headers which I attribute to the very solid "Prothane" motor mounts.
 Works for me but Sal would be dissapointed.

I get a bunch of 'blow-back'.....from my other half.
Luckily she refuses to ride in my Mustang.
She whines about "all the money you spent to make 'that Mustang'(use a disgusting voice) loud and lumpy and then...'spend all that money'  to make it quiet with insulation and stuff to stop the rattles....blah-blah-blah (I quite hearing beyond that!!....must be my age!)
 
6sal6

 Takes yor pick, both are agravating.
 Cut and pastes from Google.
Resonance,
the reinforcement or prolongation of sound by reflection from a surface or by the synchronous vibration of a neighboring object.

Drone,
to make a dull, continued, low, monotonous sound; hum; buzz.

So Steve is this something you feel or hear