Got a call from my brother yesterday, he also has a 73 GT. Anyway, he was in a bit of a state as his technical knowledge is not so good. He was on the freeway doing about 80mph, when he had a bang from the back followed by a constant brushing/grinding noise.
Went to see him last night where I only had about 5 mins to look at it. It is coming from the rear end, so I jackup the car from the diff and run it in first. I cannot judge where the noise is coming from. The bottom of the diff was quite wet with oil, but nothing unusual for these cars. No play in the wheel bearings, would say that is it the diff or torque tube. It sounds quite like a dry bearing type noise. The torque tube donut and mounts looked ok from the back of the car but could not get very close.
Before I loose a weekend sorting it for him, has anyone got any ideas. I am puzzled because in my experience, the diffs go over time and not instantly, or maybe they do?
If you can't tell where it is coming from, then my suggestion may be a long shot. But, pull the drums off the rear brakes. If a tension spring broke it might make a nice noise and then the dangling end could scrape against the drum. Hardly anyone replaces those springs and they can wear pretty thin where they hook on.
Does your brother remember what he was doing when he heard the noise? Accelerating, braking, nothing?
Now that I think about it, pull out the torque tube. If the rubber support failed or the tube mounts failed, the tube could be moving up and contacting the shaft inside.