Bladeley’s phonological loop (BL) is a mechanism to switch off language briefly. If I need to reorient myself quickly, i.e. tap into that core knowledge system which uses a limited set of information, crucially not including colour, I could disable language by filling the BL up with music. That is why the tune keeps popping into my head. Dammit. Is the earworm phenomenon a case of selective language engagement to combine core knowledge systems?
But that would seem to imply that you could only think propositionally when the BL was free, so not when a tune is in your head. hardly plausible.