Pop noise that sounded like it came from the front of the vehicle?

In this past year I had the transmission replaced, new rear brakes and rotors, new driveshaft and brackets. New wheel bearings on the right front and left rear. Recently, the shop noticed leaking gear oil from the left rear and said the axle needed to be replaced. He did that and the left front wheel bearing. After that repair, there was a pop sound that sounded like from the front. Just before that it felt like it was dragging like the brake was on and the free after the pop. This was in reverse as I had to back up the driveway to a flat area where I turned around and went up a hill forward. After I turned around there was a louder pop and a sound that sounded like a sea lion that goes away if I apply the brakes slightly. I have had it back in the shop twice and they can’t find the problem even though they can hear it. He said last time it was a loose bushing on the left rear knuckle, but it didn’t work. He said I might have to wait until it got worse before he could find the problem. I hope worse doesn’t mean something breaking or a wheel coming off. Any ideas? The guy has done a great job with everything, which has cost me big time. I don't want to end up with this.