Strange Noise/rattle

[oldmann]

I was hoping you guys could help me out with a noise I hear once in a while. It sounds like going over rumble strips when I m going 49mph @ 1450 rpm's. It is sustanable at this speed although is comes and goes when I am going slower. Has more to do with rpm range than speed. If rev it up in the driveway I can hear it come and go. Could it be a heat sheild or even a catalytic converter let go inside its housing? Its got 26k miles. I had that happen in my 06 silvy.