Comp's XE Cams

One reason I can think of that more aggressive cams [ such as XE 256/XE268 ] are making more low end tq than a stock 2bbl cam is that the XEs have [a] more lobe lift quicker opening & closing.......which gives [c] more area under the curve.

On the noise from XE flat tappet lobes.

I think Hysteric nailed it. Faster closing rate. However, there could be an additional contributing factor: they have higher lift & more area under the curve compared to other Comp lobes. This requires stronger springs for valve train control. Stronger springs will cause faster lifter bleed down...& that adds to the noise.