Remove the headlight from its nacelle and check to make sure you don't have some wires touching that shouldn't be. Also, check where the wire bundle enters the rear of the headlight nacelle and check for possible wearing through the wiring insulation causing a shorting against the nacelle. Inside you will find two 12V accessory power leads that aren't connected to anything. They should have spade connectors at the ends, with plastic or vinyl insulating covers to prevent shorting.
If you don't come up with anything there, go down to the lower front left of the bike. There you will find the voltage regulator. Remove the screws that hold the rock guard in place, then remove the unit itself. There is a plug that you can remove. It should be seated solidly. If it isn't, and it's loose, the poor connection there could cause the sound as you describe it. Vibration from both the engine and the road could be causing it to make sporadic connection. If that's it, when it loses the connection, it will cause the rpms to drop. Beyond that, take it to your dealer for warranty work as they have all the proper diagnostic instruments to track down the problem.
"Don't sweat the small stuff---it's all small stuff"
'09 650 VStrom