HT1550 XLS Buzzing/Ticking Sound on Transmit

[jdb334]

I recently got an HT1550-XLS VHF portable, and suddenly after just a few weeks of good operation it has started making a loud ticking or buzzing or humming sound when I transmit with it over the 2 meter ham band (what I mean is that the radio transmits this sound, not that I hear it while I'm transmitting). The interesting thing is that the sound goes away completely when I use the speaker mic -- it's only a problem when I use the internal mic. But this is the only way I can stop the ticking sound, by using the sp-mic -- no other setting (hi/low power, different battery, different channel, etc.) seems to stop it.
I've tinkered with a lot of the programming settings; I thought originally it might be that the gain on the mic was too high, but it is actually set lower than the speaker mic gain setting. Does anyone have ANY idea what is causing this to happen??? I would be very grateful for your help!