Resonating fans

[Vertigo]

Ok so I have three intake fans on the front of my case but I've discovered they create an annoying resonance when operating between 58% and 65%. It's most noticable around 62%.

Any ideas on how I can deal with this?

I did think of trying to alter my fan curves to put a "step" in around this range so they step 'over' the offending range. Anyone tried something similar?

 

[Vertigo]

They're all controlled together and are decent NF-A14 Noctuas.

I really don't want to start faffing with airflow again as it took me an age to get it balanced at all loads.

Not sure if it's actually reasonance or rather "beats" interference as the fans won't be running at exactly the same speed due to manufacturing tolerances.
For now I've adjusted my curves to put a 'step' from 58% to 65% so they shouldn't ever run at that speed. Will see how I get on.