Well, it could be a false alarm (pun intended) but lately I have been getting more and more rigs for repairs, and from diferent manufacturers, with bad Toko black ceramic filters (6 and 9 kHz by now)
I have fixed three rigs in the last few weeks with this problem, two Yaesu FT-897 and one Kenwood TS-2000. Symptoms could range from lower sensitivity, to cracking noises and, usually, you have both. There is also an interesting effect, at least on FT-897s, which is that they receive in DSB instead of SSB when filter fails.
I have opened one filter and it had some greenish looking residue on it, so I guess sealing had failed and moisture did the dirty job.
I would be interested in hearing other people experiences with those small "BLACK NOISIES" (*) (remember you saw it first here, (c) EB5AGV ;-) )
(*) the equivalent to "BLACK BEAUTIES" caps in old boatanchors!