Muffler bearings can only be packed with elbow grease!
Does the sound go away when you place the trany in gear and or driving, take the beauty hubcaps off, there might be one that is loose.
Also you can check the fan clutch too see that it is free when stoped and does not have any play, then take the serpentine belt off and run engine, do not run it too long! No noise then check all rotating components, alternator, power steering pump, belt tenssioner,iddler pulleys, and if equiped with air pump, check it too.
Check that the exhaust hangers are all ok, shake the pipes and see if they are O.K., also check the motor and transmission mounts if one has collapsed it might be metal to metal and/or some other part touching, also the shields to the catalitic converters, or the shield to the muffler and/or the exhaust piping.
If the cabin and the house sit on the rubber mounts attached to the frame make shure that they are not collapsed, check that the rear bumper is not loose, empty all compartments and take off any towing/hitch parts and test for noise.
Also make shure that the gray and black tanks do not move, dito for all the pipes, also check all the fuel lines and the fuel filter, if yours has a rear fuel thank make shure that it is bolted correct, also check the brake lines at the rear end and along the frame.
If you still can not locate the noise, Have a cold beer, eventually you will find it.
navegator