I noticed this on an EZY airbus back in March. Whilst we were climbing, i suddenly heard the pitch of the engine change.
I can only assume that we were cleared to climb at a higher speed (i.e 250-300KTS or 300KTS-M 0.XX).
Nothing to worry about, as Rainboe said all different noises can be heard. I too heard the Hydraulics PTU make the funny barking sound!