[ErroneousBosh]

> 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.

[buescher]

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.