What would cause an abrupt increase in engine noise after takeoff on the Boeing 737-800? I flew last week on a Boeing 737-800, I was in row 7, so just in-front of the wings. 

I know from experience that there is a significant difference in engine noise from the front to the rear of the aircraft, up front you get more of the buzz, towards the back you get more "roar" engine exhaust noise. 

However on this occasion the thing that struck me was an abrupt change in noise at about 1 minute from takeoff, not sure on altitude, but probably around 2000ft.

The takeoff (EMA) was 100% normal, straight climb no significant turn.

At this (estimated) 1 minute mark, the right-hand engine suddenly got louder, a harsher buzz, but seemingly no change in RPM (no pitch change). I would say inside the aircraft it almost doubled in volume. 
There was no obvious added vibration to the airframe.

The flight went on as normal (4 hours to ACE Lanzarote) and the noise subsided as we reached cruise. 

What could cause this? 
A few things went through my mind: undercarriage retraction(?). Bird strike, blade damage, flame-out, resonance, internal changes in the engine such as guide vane angles, engine working loose!
The buzzsaw sound is usually the fan blades breaking the sound barrier.

Could be the elevation change, temperature, and humidity caused the sound to be more pronounced at that time, or perhaps the pilots increased the thrust on that engine due to cross winds or for some other reason.

Here's a youtube video with a similar noise: