My guess would be the packs being turned on (air conditioning) after a packs off take-off giving a whooshing sound. Once in the climb the engine noise should be about constant (we set the thrust levers to the climb detent - gives us max climb thrust). If we are climbing generally we will have climb thrust. If we change the speed, the nose is lowered or raised, thrust will be the same.
(Ignoring climb in vertical speed).
Or what Rainboe said about level off.