I have the same issue with my ‘49 3100. I have checked everything except the front shocks.
I was leaning over the left fender for some reason and heard a squeaking sound from my weight on the fender. The shocks only have 7,000 miles on them, but they ain’t supposed to squeak.
My next adventure will be removing the shock to see if the squeak goes away, and replace it if does and road test it. I will report back with my findings.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission