The car doesn't have be old to develop this fault. My '85 190E started the click-click routine when it was nearly new and it was an AC component behind the glove box. Seems like it was called a changeover valve but that may be wrong. Just one of dozens of warranty claims.
Why not just remove the glove box (five minutes, tops) so you can eyeball the noisemaker for sure. It was pretty common when the 201s were new-ish. You would think that the problem would have been solved before '87. Roger Ellingson/Seattle
