Rad fans are a possibility, although I don't see how acceleration would affect noise. Speed might, due to air flow on blades from car movement. Cause could be either bearing or air flow whistle.
Pulling fan hi-speed relay (r2 - black, engine fuse box) for a short test would confirm/eliminate this source. Perform test from cold start, a/c off, relay out. Start, run, a/c on, listen, a/c off, end test, ignition off, restore relay.
If your car is still under warranty you might want to get it checked at dealer in case it's compressor related.
