> I’d naively expect the 555 to be less noisy than a clocked digital microcontroller
TTL ones were exceptionally noisy because the output transistors "shot through" - both output transistors would conduct for a moment shorting the supply rail to ground and crowbarring ridiculous interference onto other parts of the circuit.
And there's another reason not to recommend them - no one sane has used the bipolar version since the Carter administration but they're still out there and it's another pitfall for beginners.
TTL ones were exceptionally noisy because the output transistors "shot through" - both output transistors would conduct for a moment shorting the supply rail to ground and crowbarring ridiculous interference onto other parts of the circuit.