OK, I have had this problem again, only now it is getting worse.

I have verified it has nothing to do with the prox sensor or the volume control.

The last call was so bad I had to connect headphones. Applying more pressure to the phone against my ear made the sound crackle in and out. It sounded exactly like it should sound if there was a loose wire.

I will be disassembling my N900 tonight. I'll post pictures if I find anything suspicious.