rem,

You didn't introduce added pressure but with the added flow demand through the BFP you are probably hearing the noise cause by higher velocity than the fixture is designed for. Increase the size to a larger BFP and the noise should go away.
The Flow is equal to area of BFP opening times velocity through that opening. If flow rate(demand) increases the opening stays the same then the velocity has to increase. This also causes water hammer when in your system when auto valves close suddenly. WH can cause damage to valves and can cause cracks in the pipeline. Just a thought. JMHO Aloha

That surprises me that the increase flow would be more than the BFP could handle. I would think I have more flow from my regular rotary heads watering the lawn than I do on the drip line, even with it extended. Maybe the drip line uses more water than I thought it did.

The increased water demand is making the valves open and shut because you don't have enough water to maintain pressure on the inlet side of the valve. There are inherent pressure drops built in to these devices. Hopefully this advice helps 4 years late!