weird strut sound

i recently had the struts replaced in my car and lately i have been hearing an odd sound coming from the rear end, it happens every time i take off, it goes away after i get up to around 10 or 15 mph...what could it be?

1996 Olds Cutlass Supreme SL
2-Door Alloy Wheels 3.1 L V6
148,000 miles still runs great

What does this noise sound like? Are you sure you have struts in the rear and not shocks?