One reason is because water is not completely filling the flexpipe...it sorta sloshes around. Not sure how large this pipe is but you might try inserting another flexpipe inside this one to take up some of the space. The water will run along the inner flexpipe (Surface tension) and make less noise.
I use the modified durso approach (to eliminate the waterfall effect in the overflows) and then have the output of the water flows flow through a pipe that comes out under the water level in the sump - that keeps it very quiet (but my pumps and chiller are not quiet)