Yes it could be something else, most like the vibration of the mtn tread on pavement. But I couldn't say based on what you've posted. if it helps, disc rub has a certain chirpy sound, you might duplicate it by applying the brakes ever so slightly as you ride and seeing it that's what you hear or feel.

Otherwise, I'd assume the shop knows what they're doing (thought that's not always true). If you're going to ride this mainly on pavement you might consider new tires with a smooth tread, or at least a center ridge if for mixed pavement and dirt. This will not only reduce vibration, but greatly reduce the rolling resistance for an easier ride.

Pardon the dig at the shop, but if they asked (which they should have) and you said it was for pavement or commuting, they should have spoken to you about more appropriate tires and suggested a change at the time of sale.