I've tried it with battery pack in both ways, still had that same knocking though, didn't know if maybe some kind of spacer was missing? But been advised now to put a piece of foam in there to hold it in nice n snug! Will try thatIf I remember rightly if you put it so the wires are at the front it should hold it tight.
This.get a sponge, cut a piece to fit, voila!
If I remember rightly, they were first released running 6 batteries, then reduced to 4, but never changed the design. So issued a piece of foam/sponge with it.