Helicopter rotor noise question
 

I was standing outside the office the other day (open countryside) and a small helicopter flew across my field of vision about a mile away. I could mostly hear the engine noise (piston engine) and just a faint rotor beat. As I watched it, for about two seconds the sound changed to a heavy rotor beat, and then went back to normal. The aircraft didn't seem to make any change in course, attitude or speed corresponding with the change in noise.

I was just curious to see if anybody has an idea what would cause the change in noise?

Thanks very much.

The aircraft perhaps hit a gust of wind and you briefly heard an increase in blade "slap".

Duh, I never gave that a thought! Sounds very likely to have been the cause, thanks.