Sounds normal. If the noise doesn't occur if you downshift first, I'd guess it's the lugging of the engine due to the lower RPMs while "hitting the throttle hard."
You didn't say if you had a timing advancing trigger wheel or not which may also contribute to the growl as she tries to move up the power curve from a lower RPM while staying in the same gear.
I like to think of these sounds as the beast waking up.

Once you get to 3500 RPM in 5th her rumble should give way to a smoothing out and a request for more gas.