Always figured weird camera smells were just old grease or something. Then I opened up an old lens and found actual mold growing inside. That sour, cheesy smell is exactly what fungus on glass or foam can smell like. You might want to check for little fuzzy spots, it's pretty common in old gear stored in damp places.
Hold up, I gotta disagree here. Honestly that sour smell could be a ton of things besides mold. Old grease breaking down inside the shutter mechanism can get pretty rank, and some old adhesives or foam seals rot and smell exactly like funky cheese. I've had gear that stank but was totally clean inside, just decades of weird chemical changes. Unless you see actual fuzz or spots, I'd bet it's just nasty old camera guts, not fungus.