1978 corvette spindle nut
You say the castle nut was loose on the spindle and you tightened it. The castle nut should have a cotter pin running through it to prevent it from coming loose. I suspect that someone set up the rear wheel bearings in your car and possibly they didn’t do it correctly because the spindle nut should never be loose.
The run out on the spindle bearing should be between .001 and .008 (we like to run them at .003 - .004) with the nut tightened to the correct torque specifications. So I suggest that you remove the half shaft from the flange and put a dial indicator on the flange and check the run out of the spindle and bearings. Anything out of the specs above would indicate a bearing, clearance or set up wrong issue.
What you may have done when you torqued the nut is pulled the bearing in too tight and thus the noise. Another issue is backing plates can be bent and rubbing the rotor, but since you said this issue came after you torqued the nut I’d look at that first.
Willcox








