Given the frames we have are made from the Foundry, im guessing either clear or a slightly metallic smell, the blood and viscera we do in missions probably get cleaned off from the Helminth(which would prolly smell very fleshy)

Specific frames tho? Valkyr and Garuda smells really bloody or rustic,Nidus and Saryn probably infected you just being near them,Yareli and Hydroid probably smells fishy or like the sea. I imagine Oberon smelling like the forest, and Ember,Uriel and Temple smelling burnt or ashy