Glad you got it figured out. I was wondering if one of the parts underneath had shifted to cause the noise. I was wondering why it would be stiff in only one place. I had one that I thought that it was something on top since taking the thread jam didn't help. I finally figured out that indeed it was a problem with the bottom. While the link doesn't work in
my thread about that 1957 FW, it was enough to give me confidence to doing more. On that machine all the set screws across the bottom should line up. One of the rods had twisted so that the set screw didn't line up with the others. I don't think it is quite the same on the earlier machines.
Janey - Neat people never make the exciting discoveries I do.