My 1996 Subaru Legacy makes a squeak-squeak-squeak sound when it is in reverse gear; and rolling backward....if I put it in neutral while rolling; the squeak stops....any ideas what this is; and how serious it might get overtime??

What happens to the squeak if you apply the brake gently whilst reversing?

Any chance it sounds like this?

If it happens 4 times for every time a wheel turns once, almost certainly a U joint.