I was have the same problem with tri-x 320. I narrowed it down to more constant agitation, per kodak web site for this film, and changed dev from d76 to rodinal at 1:50. I had not experienced this with any other film I have shot recently (hp5, foma retropan 320, arista 400, tmax100). I have been tray processing only, and printing only, last shots with tri-x 320 in Rodinal 1:50 were free of these artifacts, although a little contrasty so I might need to exp with times/ agitation. I have been wanting to try Tri-x with Xtol dev and it is on the list.

This is my first post and I am thankful for all the wonderful information here. Thank you.
Keith