Noise is not b/c of the bias high impedance, unless you have a very special corner case, which I doubt. More probable explanation is your VB_pre is noisy. Where’s the 9V coming from?
Show the full schematics to give better advice. How’s the noise looking on an oscilloscope? Is it buzz or hum? Under what conditions do you observe the noise?
As far as capacitive loading of opamps - the opamp can go to oscillation. Google phase margin, and look for opamp rail splitters. Your 4.5V bias scenario happens in guitar amps, typical 2x10K || 100uF is fine for biasing. Phase margin issues aside, if you have noise on VB_pre the opamp will try to force in on the capacitor. It will fail at it, and the total outcome is worse than when you’d replace the unity gain opamp with a wire.