1. Try a different 50 mm lens—preferably a Leica lens.
If the issue disappears, it’s most likely a tolerance issue with the non‑Leica lens. You can also try gently twisting the lens back and forth to see if the problem changes.
2. If the issue persists, do the following:
Use a Q‑tip lightly moistened with lighter fluid. Move the viewfinder selector and look at the arrow area (see attached photo). You’ll notice one metal piece moving up and down against another. Use a Q-tip dampened with lighter fluid clean these two metal surfaces by moving the selector back and forth. Do not pour any liquid into the mechanism—only use the dampened Q‑tip.
3. If none of the above resolves the problem, your M2 is likely due for a CLA.