" Assuming a good balun, feeder in symmetry plane of beam and the 50"
under ground, I do not expect much off it.

Also when there would be noise climbing up the feeder (to the beam),
the noise could also use the (metal) mast.

Good point and that is why I am a little confused about using chokes to
lower noise --- in any situation where the coax goes up a tower.

A number of very well-written and interesting papers
describe the effective rejection of chokes (over 5000 ohms when looping the
coax
through several #31 toroids), but they do not discuss the effects on the
total antenna system. As you said, it would seem that any pickup by the coax
external shield would be mostly duplicated by the tower itself and this
would bypass the choke if the coax shield is "grounded" to the tower at the
feed point. (Yes, I know "ground" has many meanings.)

I can see where the choke would help with a dipole, where the coax is not
connected to the tower at all.

Bill - W2WO