Tu-154 Thrust Reduction on Takeoff
 

I've noticed this on all the Tu154 videos I've watched, and when I flew on one recently was waiting for, and heard/felt the exact same thing.

When the aircraft starts its takeoff roll, after a few seconds the thrust reduces temporarily, then rises again. I've watched flight deck videos and never been able to see what it is that causes the engines to throttle back, and why it's done.

You can hear it quite clearly in this video (takeoff is at the 22 minute mark):

Can anyone shed any light as to what this is?

Igniting the afterburners? (Only joking, but this used to happen with some marks of Lightning)

Personnally I don't think the noise you refer to indicates a thrust reduction. Could be caused by any number of things such as airflow through the engines (witness the rising and falling howl the Vulcan makes at the start of it's takeoff run). What we need is a Tu154 rated pilot to come along and solve your query.