Pinion angle

Did you have the sfc's welded in when the cars suspension was loaded or unloaded? The suspension needs to be loaded when putting those in.

Yeah your cars suspension needs to be under load when welding subframes in, that could make the difference.

BS mine was on the hoist when I welded/bolted mine on.

When welding in subframes the car MUST have all for tires on level ground, with then weight of the vehicle on them. This is best done on an alignment rack or 4 post drive on lift. If you do it on the 2 poster you run a big risk of the body/subframe being tweaked, and when welded it says in this position which will throw a lot of **** off. More so door jams and latches than anything else. I dont see this causing a grinding noise. There is a chance the pinion could be off, is the car lowered? Do you have adjustable control arms? If there was a problem in the rear axle you would notice alot of other problems other than what you have. You would also have a gear whine and or clunking noise on drive or coast situations. Give me some more info about exactly when it happens and what other mods are done with the steering/suspension and brakes. If someone tells you its a good idea to weld in subframes any other way is an idiot. Rack a older unibody car on a 2 post lift that may not be dead nuts even and open the doors, or hell a convertible for that matter. Then open the door and try to shut it. It may work but its going to feel a little different than when its on all 4 wheels. Hell most convertible and T-top Vettes are suppose to have the doors open before lifting. This is all due to unibody flex, dont believe me call a chevy dealer ask the service manager about racking a Vette.