Droning sound when car accelerates

[csundaram_175091]

we have an 03 camry Automatic, well maintained and having maybe 40K miles on it. Recently we have observed that when the car first accelerates from slow speeds it makes a low droning sound (drrrrrr).When it reaches higher speeds the droning sound disappears. We took the car for its regular servicing at which point we were told the center didn’t exactly know what was causing it but it MIGHT be the intake manifold. We took it to a Midas for a second opinion, and the guy there was not convinced it was the manifold and could not identify what might be causing the sound. Any ideas? have you come across this with other camrys orin your own car and what was the diagnosis? Much thanks for advice.