Problems with Autofocus

[ClassicBeth]

My Canon 30D has been having issues with focusing lately. I tested it a while back and found that it was backfocusing with my 24-105 and my 17-55. I figured I should send in the whole lot, but Canon told me to just send the body. They reported that it was, indeed, backfocusing, charged me $200 to adjust, and sent it back to me. I tested it right away and it was still backfocusing with my lenses, so I sent everything back in. Canon said that I was right, the lenses were backfocusing too. They adjusted the lenses "as a courtesy", meaning, they didn't charge me more but they would not warranty the repair either. Everything came back and focused where it should, so they did fix the backfocus problem.

Here's the new issue: It's been about a month since I got the camera back and since then it is hunting for focus a LOT, and in difficult light it sometimes won't find a focus at all. I rarely encountered that before. Something else also has me worried, and that is that my 24-105 now makes a squeal type noise when is trying to focus. It's like a high-pitched screech, not very loud, but definitely there. It did not make that sound before this round of "repairs". The other zoom doesn't make the sound, but it also has trouble finding a focus. I have run into the not focusing problem with my 50mm (1.8) as well. The other night I was trying to take pictures from the audience at a church concert, and granted the light was horrible, but only a couple out of maybe 60 pictures actually were in focus. When I reviewed the pictures I figured they just looked mushy because of ISO 1600, but then I got to one where the focus actually *worked* and there was a big difference. The settings were the same. Still not the greatest picture ever, but at least the guy's face looked relatively clear and sharp.

What is going on here? Is the focus motor in trouble? I'm getting really tired of sending my camera away for weeks at a time, not to mention how expensive it is to ship insured. I'm hoping that maybe it's just dirt on the connections. Can anyone describe for me exactly what parts of the camera and lenses to clean? I don't want to buy a new camera yet, especially since I just spent nearly 250 bucks to ship and repair the thing! And I'm really worried about the 24-105. Help, please!