Well, I've never fostered, but we have had similar experiences in the past. Kuhio was sweet and loving to us. She'd be purring her brains out and if the doorbell rang or the phone, she'd take off like she'd been lit on fire and hide until the awful noise and people left. We started leaving talk radio on all day while we were at work. At first we'd have to track her down when we got home. Eventually she got used to the noise and voices and we'd find her out in the open when we got home. We don't get many visitors either and our house is pretty quiet. Leaving the radio or TV on could be worth a try.