For front shifting, electronic is quite a bit louder.

For rear, I'd say that mechanical is probably louder, but close. Shimano mechanical seems quieter than SRAM mechanical.

for me, electronic has been markedly quieter. i was just thinking about this yesterday on a long ride. i've had it for 1.5 years and still really like it.
peggy

I think it depends more on the chain and cassette than the shifting mechanism (derailleur)

Shifting to a smaller cog in the rear should be the fastest/quietest shift you can do (independent of drivetrain). Basic rule of attacking is to shift well ahead of the attack specifically to prevent telegraphing.

Big, hollow tubes and wheels definitely amplify drivetrain noise, and the old SRAM powerdome cassettes were incredibly loud.

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Clonk! Clonk! Whooooohohohooooooooowoooooo! Clonk! Krrr krrrr! (larger sprockets)

That's me with SRAM RED and Zipp Firecrest rimbrakes. :) Everyone hears me coming.