Fist off, try and observe EXACTLY what's happening when you pedal under load. Often periodic or pulsed grinding is the result of the chainrings moving outward when the right crank is loaded.
If you see the movement, the cause could be frame flex at the BB, or a slightly loose BB, or even crank flex.
In any case the diagnostic process starts with observing what's happening, then looking for a possible cause.
BTW- some chainring movement under load is pretty normal, so if it's only slight, the answer might be to trim the FD outward a bit to accommodate it.
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