New exhaust bad vibration reverse

bought new 3 inch RJC exhaust
I could not install required pro and some minor welding
Under car does not appear to hit anything and clearance looks ok

When not in forward gear loud noises (rattles vibration) like somethings falling apart underneath
When in drive or moving forward don't hear and feel it
Reverse is much worse than neutral but occurs in neutral also

Anyone had this issue before?

With the car just sitting there, engine off, grab the tail pipe and shake it. Does the exhaust rattle? If so it is hitting someplace.