From the sounds of it, you can try under the carpet. Then, you're only left with removing the drywall. Install a temporary dehumidifier and wait long enough to see if the mold/mildew smell returns. IF it does, there's moisture back there because the dehumidifier SHOULD remove the excess moisture in the room; but maybe NOT able to remove it from BEHIND the wall. Put the unit NEAR the affected area, and by the amount of water you 'catch', you'll know if the house is overly humid.
Assuming the dehumidifier test doesn't help, and the mold/mildew returns, then it must be 'growing' where you can't see it. Again, understand, that in order for this stuff to 'thrive', there needs to be a relatively high temperature and a constant 'source' of water/moisture/humidity. Eliminating or dramatically reducing EITHER of these 2 conditions, your problem SHOULD go away. If anything I've said to date isn't clear, ask away. Perhaps you need to e-mail me, directly, because you post is beginning to get 'buried'.
For now, I do wish you well. My best to ya and hope this helps.