Rattling when giving gas and letting go of gas pedal

Have a 2012 subaru legacy and when I give gas I hear like a "bell" type sound. Like a "bell"or "rattle"type sound; It occurs mostly on speeds under 60 and mostly when you give it gas. If you quickly give gas and then let go you can hear it make the sound. I was told it probably was the belt tensioner, does this sound like good advice?