Squeak underneath car when in reverse

Put it into reverse and it squeaks rhythmically upon moving, with one little pop/tap/knock when speeding up or slowing down a bit, but not when the car is still. Sound comes from under passenger seat undercarriage. It continues when put into first, but quickly goes away. My mechanic is booked until August and he had a listen he thought it was dust in the brakes (not coming from tire location though) or the rear drive-train. I will have it looked at in August, but in the meantime, was planning a fourteen day vacation drive 1,800 miles max all told. Now this full time single mom is worried I will get stranded all five of us (two kids, two dogs and myself!).